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In embedded mode, an application opens a database from within the same JVM using JDBC. This is the fastest and easiest connection mode. The disadvantage is that a database may only be open in one virtual machine (and class loader) at any time. As in all modes, both persistent and in-memory databases are supported. There is no limit on the number of database open concurrently, or on the number of open connections. Show
In embedded mode I/O operations can be performed by application's threads that execute a SQL command. The application may not interrupt these threads, it can lead to database corruption, because JVM closes I/O handle during thread interruption. Consider other ways to control execution of your application. When interrupts are possible the file system can be used as a workaround, but full safety is not guaranteed. It's recommended to use the client-server model instead, the client side may interrupt own threads. Server ModeWhen using the server mode (sometimes called remote mode or client/server mode), an application opens a database remotely using the JDBC or ODBC API. A server needs to be started within the same or another virtual machine, or on another computer. Many applications can connect to the same database at the same time, by connecting to this server. Internally, the server process opens the database(s) in embedded mode. The server mode is slower than the embedded mode, because all data is transferred over TCP/IP. As in all modes, both persistent and in-memory databases are supported. There is no limit on the number of database open concurrently per server, or on the number of open connections. Mixed ModeThe mixed mode is a combination of the embedded and the server mode. The first application that connects to a database does that in embedded mode, but also starts a server so that other applications (running in different processes or virtual machines) can concurrently access the same data. The local connections are as fast as if the database is used in just the embedded mode, while the remote connections are a bit slower. The server can be started and stopped from within the application (using the server API), or automatically (automatic mixed mode). When using the , all clients that want to connect to the database (no matter if it's an local or remote connection) can do so using the exact same database URL. Database URL OverviewThis database supports multiple connection modes and connection settings. This is achieved using different database URLs. Settings in the URLs are not case sensitive. TopicURL Format and Examples jdbc:h2:[file:][ Connecting to an Embedded (Local) DatabaseThe database URL for connecting to a local database is SET DB_CLOSE_DELAY 10 1. The prefix SET DB_CLOSE_DELAY 10 2 is optional. If no or only a relative path is used, then the current working directory is used as a starting point. The case sensitivity of the path and database name depend on the operating system, however it is recommended to use lowercase letters only. The database name must be at least three characters long (a limitation of SET DB_CLOSE_DELAY 10 3). The database name must not contain a semicolon. To point to the user home directory, use SET DB_CLOSE_DELAY 10 4, as in: SET DB_CLOSE_DELAY 10 5. In-Memory DatabasesFor certain use cases (for example: rapid prototyping, testing, high performance operations, read-only databases), it may not be required to persist data, or persist changes to the data. This database supports the in-memory mode, where the data is not persisted. In some cases, only one connection to a in-memory database is required. This means the database to be opened is private. In this case, the database URL is SET DB_CLOSE_DELAY 10 6 Opening two connections within the same virtual machine means opening two different (private) databases. Sometimes multiple connections to the same in-memory database are required. In this case, the database URL must include a name. Example: SET DB_CLOSE_DELAY 10 7. Accessing the same database using this URL only works within the same virtual machine and class loader environment. To access an in-memory database from another process or from another computer, you need to start a TCP server in the same process as the in-memory database was created. The other processes then need to access the database over TCP/IP or TLS, using a database URL such as: SET DB_CLOSE_DELAY 10 8. By default, closing the last connection to a database closes the database. For an in-memory database, this means the content is lost. To keep the database open, add SET DB_CLOSE_DELAY 10 9 to the database URL. To keep the content of an in-memory database as long as the virtual machine is alive, use String url = "jdbc:h2:~/test;DB_CLOSE_ON_EXIT=FALSE"; 0. This may create a memory leak, when you need to remove the database, use the command. Database Files EncryptionThe database files can be encrypted. Three encryption algorithms are supported:
To use file encryption, you need to specify the encryption algorithm (the 'cipher') and the file password (in addition to the user password) when connecting to the database. Creating a New Database with File EncryptionBy default, a new database is automatically created if it does not exist yet when the url is used. To create an encrypted database, connect to it as it would already exist locally using the embedded URL. Connecting to an Encrypted DatabaseThe encryption algorithm is set in the database URL, and the file password is specified in the password field, before the user password. A single space separates the file password and the user password; the file password itself may not contain spaces. File passwords and user passwords are case sensitive. Here is an example to connect to a password-encrypted database: String url = "jdbc:h2:~/test;CIPHER=AES"; String user = "sa"; String pwds = "filepwd userpwd"; conn = DriverManager.
Encrypting or Decrypting a DatabaseTo encrypt an existing database, use the String url = "jdbc:h2:~/test;DB_CLOSE_ON_EXIT=FALSE"; 1 tool. This tool can also decrypt an encrypted database, or change the file encryption key. The tool is available from within the H2 Console in the tools section, or you can run it from the command line. The following command line will encrypt the database String url = "jdbc:h2:~/test;DB_CLOSE_ON_EXIT=FALSE"; 2 in the user home directory with the file password String url = "jdbc:h2:~/test;DB_CLOSE_ON_EXIT=FALSE"; 3 and the encryption algorithm AES: java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd Database File LockingWhenever a database is opened, a lock file is created to signal other processes that the database is in use. If database is closed, or if the process that opened the database terminates, this lock file is deleted. The following file locking methods are implemented:
To open the database with a different file locking method, use the parameter String url = "jdbc:h2:~/test;DB_CLOSE_ON_EXIT=FALSE"; 9. The following code opens the database with the 'socket' locking method: String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; For more information about the algorithms, see . Opening a Database Only if it Already ExistsBy default, when an application calls String url = "jdbc:h2:mem:test;INIT=runscript from '/create.sql'\;runscript from '/init.sql'"; 0 with URL and the database specified in the URL does not yet exist, a new (empty) database is created. In some situations, it is better to restrict creating new databases, and only allow to open existing databases. To do this, add String url = "jdbc:h2:mem:test;INIT=runscript from '/create.sql'\;runscript from '/init.sql'"; 1 to the database URL. In this case, if the database does not already exist, an exception is thrown when trying to connect. The connection only succeeds when the database already exists. The complete URL may look like this: String url = "jdbc:h2:/data/sample;IFEXISTS=TRUE"; Closing a DatabaseDelayed Database ClosingUsually, a database is closed when the last connection to it is closed. In some situations this slows down the application, for example when it is not possible to keep at least one connection open. The automatic closing of a database can be delayed or disabled with the SQL statement String url = "jdbc:h2:mem:test;INIT=runscript from '/create.sql'\;runscript from '/init.sql'"; 2. The parameter SET DB_CLOSE_DELAY 10 The value -1 means the database is not closed automatically. The value 0 is the default and means the database is closed when the last connection is closed. This setting is persistent and can be set by an administrator only. It is possible to set the value in the database URL: String url = "jdbc:h2:mem:test;INIT=runscript from '/create.sql'\;runscript from '/init.sql'"; 3. Don't Close a Database when the VM ExitsBy default, a database is closed when the last connection is closed. However, if it is never closed, a persistent database is closed when the virtual machine exits normally, using a shutdown hook. In some situations, the database should not be closed in this case, for example because the database is still used at virtual machine shutdown (to store the shutdown process in the database for example). For those cases, the automatic closing of the database can be disabled in the database URL. The first connection (the one that is opening the database) needs to set the option in the database URL (it is not possible to change the setting afterwards). The database URL to disable database closing on exit is: String url = "jdbc:h2:~/test;DB_CLOSE_ON_EXIT=FALSE"; Warning: when database closing on exit is disabled, an application must execute the command by itself in its own shutdown hook after completion of all operations with database to avoid data loss and should not try to establish new connections to database after that. Execute SQL on ConnectionSometimes, particularly for in-memory databases, it is useful to be able to execute DDL or DML commands automatically when a client connects to a database. This functionality is enabled via the INIT property. Note that multiple commands may be passed to INIT, but the semicolon delimiter must be escaped, as in the example below. String url = "jdbc:h2:mem:test;INIT=runscript from '/create.sql'\;runscript from '/init.sql'"; Please note the double backslash is only required in a Java or properties file. In a GUI, or in an XML file, only one backslash is required: Backslashes within the init script (for example within a runscript statement, to specify the folder names in Windows) need to be escaped as well (using a second backslash). It might be simpler to avoid backslashes in folder names for this reason; use forward slashes instead. Ignore Unknown SettingsSome applications (for example OpenOffice.org Base) pass some additional parameters when connecting to the database. Why those parameters are passed is unknown. The parameters String url = "jdbc:h2:mem:test;INIT=runscript from '/create.sql'\;runscript from '/init.sql'"; 4 and String url = "jdbc:h2:mem:test;INIT=runscript from '/create.sql'\;runscript from '/init.sql'"; 5 are such examples; they are simply ignored to improve the compatibility with OpenOffice.org. If an application passes other parameters when connecting to the database, usually the database throws an exception saying the parameter is not supported. It is possible to ignored such parameters by adding String url = "jdbc:h2:mem:test;INIT=runscript from '/create.sql'\;runscript from '/init.sql'"; 6 to the database URL. Changing Other Settings when Opening a ConnectionIn addition to the settings already described, other database settings can be passed in the database URL. Adding String url = "jdbc:h2:mem:test;INIT=runscript from '/create.sql'\;runscript from '/init.sql'"; 7 at the end of a database URL is the same as executing the statement String url = "jdbc:h2:mem:test;INIT=runscript from '/create.sql'\;runscript from '/init.sql'"; 8 just after connecting. For a list of supported settings, see SQL Grammar or the DbSettings javadoc. Custom File Access ModeUsually, the database opens the database file with the access mode String url = "jdbc:h2:mem:test;INIT=runscript from '/create.sql'\;runscript from '/init.sql'"; 9, meaning read-write (except for read only databases, where the mode 0 is used). To open a database in read-only mode if the database file is not read-only, use 1. Also supported are 2 and 3. This setting must be specified in the database URL: String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws"; For more information see . On many operating systems the access mode 2 does not guarantee that the data is written to the disk. Multiple ConnectionsOpening Multiple Databases at the Same TimeAn application can open multiple databases at the same time, including multiple connections to the same database. The number of open database is only limited by the memory available. Multiple Connections to the Same Database: Client/ServerIf you want to access the same database at the same time from different processes or computers, you need to use the client / server mode. In this case, one process acts as the server, and the other processes (that could reside on other computers as well) connect to the server via TCP/IP (or TLS over TCP/IP for improved security). Multithreading SupportThis database is multithreading-safe. If an application is multi-threaded, it does not need to worry about synchronizing access to the database. An application should normally use one connection per thread. This database synchronizes access to the same connection, but other databases may not do this. To get higher concurrency, you need to use multiple connections. An application can use multiple threads that access the same database at the same time. Threads that use different connections can use the database concurrently. Locking, Lock-Timeout, DeadlocksUsually, 5 statements will generate read locks. This includes subqueries. Statements that modify data use write locks on the modified rows. It is also possible to issue write locks without modifying data, using the statement 6. Data definition statements may issue exclusive locks on tables. The statements 7 and 8 releases all open locks. The commands 9 and String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws"; 0 don't affect locks. The locks are also released when the autocommit mode changes, and for connections with autocommit set to true (this is the default), locks are released after each statement. The following statements generate locks: Type of Lock SQL Statement Read SELECT * FROM TEST; CALL SELECT MAX(ID) FROM TEST; SCRIPT; Write (row-level) SELECT * FROM TEST WHERE 1=0 FOR UPDATE; Write (row-level) INSERT INTO TEST VALUES(1, 'Hello'); INSERT INTO TEST SELECT * FROM TEST; UPDATE TEST SET NAME='Hi'; DELETE FROM TEST; Exclusive ALTER TABLE TEST ...; CREATE INDEX ... ON TEST ...; DROP INDEX ...; The number of seconds until a lock timeout exception is thrown can be set separately for each connection using the SQL command String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws"; 1. The initial lock timeout (that is the timeout used for new connections) can be set using the SQL command String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws"; 2. The default lock timeout is persistent. Database File LayoutThe following files are created for persistent databases: File NameDescriptionNumber of Files test.mv.db Database file. Contains the transaction log, indexes, and data for all tables. Format: String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws"; 3 1 per database test.newFile Temporary file for database compaction. Contains the new MVStore file. Format: String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws"; 4 0 or 1 per database test.tempFile Temporary file for database compaction. Contains the temporary MVStore file. Format: String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws"; 5 0 or 1 per database test.lock.db Database lock file. Automatically (re-)created while the database is in use. Format: String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws"; 6 1 per database (only if in use) test.trace.db Trace file (if the trace option is enabled). Contains trace information. Format: String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws"; 7 Renamed to String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws"; 8 if too big. 0 or 1 per database test.123.temp.db Temporary file. Contains a temporary blob or a large result set. Format: String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws"; 9 1 per object Moving and Renaming Database FilesDatabase name and location are not stored inside the database files. While a database is closed, the files can be moved to another directory, and they can be renamed as well (as long as all files of the same database start with the same name and the respective extensions are unchanged). As there is no platform specific data in the files, they can be moved to other operating systems without problems. BackupWhen the database is closed, it is possible to backup the database files. To backup data while the database is running, the SQL commands jdbc:h2:/data/test;AUTO_SERVER=TRUE 0 and jdbc:h2:/data/test;AUTO_SERVER=TRUE 1 can be used. Logging and RecoveryWhenever data is modified in the database and those changes are committed, the changes are written to the transaction log (except for in-memory objects). The changes to the main data area itself are usually written later on, to optimize disk access. If there is a power failure, the main data area is not up-to-date, but because the changes are in the transaction log, the next time the database is opened, the changes are re-applied automatically. CompatibilityAll database engines behave a little bit different. Where possible, H2 supports the ANSI SQL standard, and tries to be compatible to other databases. There are still a few differences however: In MySQL text columns are case insensitive by default, while in H2 they are case sensitive. However H2 supports case insensitive columns as well. To create the tables with case insensitive texts, append jdbc:h2:/data/test;AUTO_SERVER=TRUE 2 to the database URL (example: jdbc:h2:/data/test;AUTO_SERVER=TRUE 3). Compatibility ModesFor certain features, this database can emulate the behavior of specific databases. However, only a small subset of the differences between databases are implemented in this way. Here is the list of currently supported modes and the differences to the regular mode: REGULAR Compatibility modeThis mode is used by default.
STRICT Compatibility ModeTo use the STRICT mode, use the database URL jdbc:h2:/data/test;AUTO_SERVER=TRUE 4 or the SQL statement jdbc:h2:/data/test;AUTO_SERVER=TRUE 5. In this mode some deprecated features are disabled. If your application or library uses only the H2 or it generates different SQL for different database systems it is recommended to use this compatibility mode in unit tests to reduce possibility of accidental misuse of such features. This mode cannot be used as SQL validator, however. It is not recommended to enable this mode in production builds of libraries, because this mode may become more restrictive in future releases of H2 that may break your library if it will be used together with newer version of H2.
LEGACY Compatibility ModeTo use the LEGACY mode, use the database URL jdbc:h2:/data/test;AUTO_SERVER=TRUE 6 or the SQL statement jdbc:h2:/data/test;AUTO_SERVER=TRUE 7. In this mode some compatibility features for applications written for H2 1.X are enabled. This mode doesn't provide full compatibility with H2 1.X.
DB2 Compatibility ModeTo use the IBM DB2 mode, use the database URL jdbc:h2:/data/test;AUTO_SERVER=TRUE 8 or the SQL statement jdbc:h2:/data/test;AUTO_SERVER=TRUE 9.
Derby Compatibility ModeTo use the Apache Derby mode, use the database URL java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 03 or the SQL statement java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 04.
HSQLDB Compatibility ModeTo use the HSQLDB mode, use the database URL java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 10 or the SQL statement java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 11.
MS SQL Server Compatibility ModeTo use the MS SQL Server mode, use the database URL java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 12. Do not change value of DATABASE_TO_LOWER and CASE_INSENSITIVE_IDENTIFIERS after creation of database.
MariaDB Compatibility ModeTo use the MariaDB mode, use the database URL java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 21. When case-insensitive identifiers are needed append java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 22 to URL. Do not change value of DATABASE_TO_LOWER after creation of database.
Text comparison in MariaDB is case insensitive by default, while in H2 it is case sensitive (as in most other databases). H2 does support case insensitive text comparison, but it needs to be set separately, using java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 27. This affects comparison using java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 28. MySQL Compatibility ModeTo use the MySQL mode, use the database URL java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 29. When case-insensitive identifiers are needed append java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 22 to URL. Do not change value of DATABASE_TO_LOWER after creation of database.
Text comparison in MySQL is case insensitive by default, while in H2 it is case sensitive (as in most other databases). H2 does support case insensitive text comparison, but it needs to be set separately, using java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 27. This affects comparison using java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 28. Oracle Compatibility ModeTo use the Oracle mode, use the database URL java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 37 or the SQL statement java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 38.
PostgreSQL Compatibility ModeTo use the PostgreSQL mode, use the database URL java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 45. Do not change value of DATABASE_TO_LOWER after creation of database.
Auto-ReconnectThe auto-reconnect feature causes the JDBC driver to reconnect to the database if the connection is lost. The automatic re-connect only occurs when auto-commit is enabled; if auto-commit is disabled, an exception is thrown. To enable this mode, append java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 53 to the database URL. Re-connecting will open a new session. After an automatic re-connect, variables and local temporary tables definitions (excluding data) are re-created. The contents of the system table java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 54 contains all client side state that is re-created. If another connection uses the database in exclusive mode (enabled using java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 55 or java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 56), then this connection will try to re-connect until the exclusive mode ends. Automatic Mixed ModeMultiple processes can access the same database without having to start the server manually. To do that, append java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 57 to the database URL. You can use the same database URL independent of whether the database is already open or not. This feature doesn't work with in-memory databases. Example database URL: jdbc:h2:/data/test;AUTO_SERVER=TRUE Use the same URL for all connections to this database. Internally, when using this mode, the first connection to the database is made in embedded mode, and additionally a server is started internally (as a daemon thread). If the database is already open in another process, the server mode is used automatically. The IP address and port of the server are stored in the file java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 58, that's why in-memory databases can't be supported. The application that opens the first connection to the database uses the embedded mode, which is faster than the server mode. Therefore the main application should open the database first if possible. The first connection automatically starts a server on a random port. This server allows remote connections, however only to this database (to ensure that, the client reads java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 58 file and sends the random key that is stored there to the server). When the first connection is closed, the server stops. If other (remote) connections are still open, one of them will then start a server (auto-reconnect is enabled automatically). All processes need to have access to the database files. If the first connection is closed (the connection that started the server), open transactions of other connections will be rolled back (this may not be a problem if you don't disable autocommit). Explicit client/server connections (using java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 60 or java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd
Here is an example how to use this mode. Application 1 and 2 are not necessarily started on the same computer, but they need to have access to the database files. Application 1 and 2 are typically two different processes (however they could run within the same process). java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 0 When using this feature, by default the server uses any free TCP port. The port can be set manually using java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 62. Page SizeThe page size for new databases is 4 KiB (4096 bytes), unless the page size is set explicitly in the database URL using java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 63 when the database is created. The page size of existing databases can not be changed, so this property needs to be set when the database is created. The page size of encrypted databases must be a multiple of 4096 (4096, 8192, …). Using the Trace OptionsTo find problems in an application, it is sometimes good to see what database operations where executed. This database offers the following trace features:
Trace OptionsThe simplest way to enable the trace option is setting it in the database URL. There are two settings, one for java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 64 ( java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd
java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 68). The trace levels are 0 for java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 69, 1 for java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 70 (the default), 2 for java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 71, and 3 for java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 72. A database URL with both levels set to java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 72 is: java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 1 The trace level can be changed at runtime by executing the SQL command java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 74 (for java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 64 tracing) or java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 76 (for file tracing). Example: java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 2 Setting the Maximum Size of the Trace FileWhen using a high trace level, the trace file can get very big quickly. The default size limit is 16 MB, if the trace file exceeds this limit, it is renamed to java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 77 and a new file is created. If another such file exists, it is deleted. To limit the size to a certain number of megabytes, use java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 78. Example: java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 3 Java Code GenerationWhen setting the trace level to java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 71 or java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 72, Java source code is generated as well. This simplifies reproducing problems. The trace file looks like this: java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 4 To filter the Java source code, use the java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 81 tool as follows: java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 5 The generated file java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 82 will contain the Java source code. The generated source code may be too large to compile (the size of a Java method is limited). If this is the case, the source code needs to be split in multiple methods. The password is not listed in the trace file and therefore not included in the source code. Using Other Logging APIsBy default, this database uses its own native 'trace' facility. This facility is called 'trace' and not 'log' within this database to avoid confusion with the transaction log. Trace messages can be written to both file and java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 64. In most cases, this is sufficient, however sometimes it is better to use the same facility as the application, for example Log4j. To do that, this database support SLF4J. SLF4J is a simple facade for various logging APIs and allows to plug in the desired implementation at deployment time. SLF4J supports implementations such as Logback, Log4j, Jakarta Commons Logging (JCL), Java logging, x4juli, and Simple Log. To enable SLF4J, set the file trace level to 4 in the database URL: java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 6 Changing the log mechanism is not possible after the database is open, that means executing the SQL statement java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 84 when the database is already open will not have the desired effect. To use SLF4J, all required jar files need to be in the classpath. The logger name is java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 85. If it does not work, check the file String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws"; 7 for error messages. Read Only DatabasesIf the database files are read-only, then the database is read-only as well. It is not possible to create new tables, add or modify data in this database. Only 5 and java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 88 statements are allowed. To create a read-only database, close the database. Then, make the database file read-only. When you open the database now, it is read-only. There are two ways an application can find out whether database is read-only: by calling java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 89 or by executing the SQL statement java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 90. Using the 0 the database can also be opened in read-only mode, even if the database file is not read only. Read Only Databases in Zip or Jar FileTo create a read-only database in a zip file, first create a regular persistent database, and then create a backup. The database must not have pending changes, that means you need to close all connections to the database first. To speed up opening the read-only database and running queries, the database should be closed using java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 92. If you are using a database named String url = "jdbc:h2:~/test;DB_CLOSE_ON_EXIT=FALSE"; 2, an easy way to create a zip file is using the java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 94 tool. You can start the tool from the command line, or from within the H2 Console (Tools - Backup). Please note that the database must be closed when the backup is created. Therefore, the SQL statement java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 95 can not be used. When the zip file is created, you can open the database in the zip file using the following database URL: java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 7 Databases in zip files are read-only. The performance for some queries will be slower than when using a regular database, because random access in zip files is not supported (only streaming). How much this affects the performance depends on the queries and the data. The database is not read in memory; therefore large databases are supported as well. The same indexes are used as when using a regular database. If the database is larger than a few megabytes, performance is much better if the database file is split into multiple smaller files, because random access in compressed files is not possible. See also the sample application ReadOnlyDatabaseInZip. Opening a Corrupted DatabaseIf a database cannot be opened because the boot info (the SQL script that is run at startup) is corrupted, then the database can be opened by specifying a database event listener. The exceptions are logged, but opening the database will continue. Generated Columns (Computed Columns) / Function Based IndexEach column is either a base column or a generated column. A generated column is a column whose value is calculated before storing and cannot be assigned directly. The formula is evaluated when the row is inserted, and re-evaluated every time the row is updated. One use case is to automatically update the last-modification time: java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 8 Function indexes are not directly supported by this database, but they can be emulated by using generated columns. For example, if an index on the upper-case version of a column is required, create a generated column with the upper-case version of the original column, and create an index for this column: java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 9 When inserting data, it is not required (and not allowed) to specify a value for the upper-case version of the column, because the value is generated. But you can use the column when querying the table: String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 0 Multi-Dimensional IndexesA tool is provided to execute efficient multi-dimension (spatial) range queries. This database does not support a specialized spatial index (R-Tree or similar). Instead, the B-Tree index is used. For each record, the multi-dimensional key is converted (mapped) to a single dimensional (scalar) value. This value specifies the location on a space-filling curve. Currently, Z-order (also called N-order or Morton-order) is used; Hilbert curve could also be used, but the implementation is more complex. The algorithm to convert the multi-dimensional value is called bit-interleaving. The scalar value is indexed using a B-Tree index (usually using a generated column). The method can result in a drastic performance improvement over just using an index on the first column. Depending on the data and number of dimensions, the improvement is usually higher than factor 5. The tool generates a SQL query from a specified multi-dimensional range. The method used is not database dependent, and the tool can easily be ported to other databases. For an example how to use the tool, please have a look at the sample code provided in java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 96. User-Defined Functions and Stored ProceduresIn addition to the built-in functions, this database supports user-defined Java functions. In this database, Java functions can be used as stored procedures as well. A function must be declared (registered) before it can be used. A function can be defined using source code, or as a reference to a compiled class that is available in the classpath. By default, the function aliases are stored in the current schema. Referencing a Compiled MethodWhen referencing a method, the class must already be compiled and included in the classpath where the database is running. Only static Java methods are supported; both the class and the method must be public. Example Java class: String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 1 The Java function must be registered in the database by calling java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 97: String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 2 For a complete sample application, see java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 98. Declaring Functions as Source CodeWhen defining a function alias with source code, the database tries to compile the source code using the Java compiler (the class java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd
String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 00 is run as a separate process. Only the source code is stored in the database; the class is compiled each time the database is re-opened. Source code can be passed as dollar quoted text ( String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET";
String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 3 By default, the three packages String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 02 are imported. The method name ( String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 03 in the example above) is ignored. Method overloading is not supported when declaring functions as source code, that means only one method may be declared for an alias. If different import statements are required, they must be declared at the beginning and separated with the tag String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 04: String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 4 The following template is used to create a complete Java class: String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 5 Method OverloadingMultiple methods may be bound to a SQL function if the class is already compiled and included in the classpath. Each Java method must have a different number of arguments. Method overloading is not supported when declaring functions as source code. Function Data Type MappingFunctions that accept non-nullable parameters such as String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 05 will not be called if one of those parameters is java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 08. Instead, the result of the function is java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 08. If the function should be called if a parameter is java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 08, you need to use String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 09 instead. SQL types are mapped to Java classes and vice-versa as in the JDBC API. For details, see Data Types. There are a few special cases: String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 10 is mapped to String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 11 (a serialized object). Therefore, String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 10 can not be used to match all SQL types (matching all SQL types is not supported). The second special case is String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 13: arrays of any class are mapped to String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 14. Objects of type String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 15 (the internal value class) are passed through without conversion. Functions That Require a ConnectionIf the first parameter of a Java function is a String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 16, then the connection to database is provided. This connection does not need to be closed before returning. When calling the method from within the SQL statement, this connection parameter does not need to be (can not be) specified. Functions Throwing an ExceptionIf a function throws an exception, then the current statement is rolled back and the exception is thrown to the application. SQLException are directly re-thrown to the calling application; all other exceptions are first converted to a SQLException. Functions Returning a Result SetFunctions may returns a result set. Such a function can be called with the java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 88 statement: String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 6 Using SimpleResultSetA function can create a result set using the String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 18 tool: String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 7 Using a Function as a TableA function that returns a result set can be used like a table. However, in this case the function is called at least twice: first while parsing the statement to collect the column names (with parameters set to java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd 02 where not known at compile time). And then, while executing the statement to get the data (maybe multiple times if this is a join). If the function is called just to get the column list, the URL of the connection passed to the function is String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 20. Otherwise, the URL of the connection is String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 21. String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 8 Pluggable or User-Defined TablesFor situations where you need to expose other data-sources to the SQL engine as a table, there are "pluggable tables". For some examples, have a look at the code in String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 22. In order to create your own TableEngine, you need to implement the String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 23 interface e.g. something like this: String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 9 and then create the table from SQL like this: String url = "jdbc:h2:/data/sample;IFEXISTS=TRUE"; 0 It is also possible to pass in parameters to the table engine, like so: String url = "jdbc:h2:/data/sample;IFEXISTS=TRUE"; 1 In which case the parameters are passed down in the tableEngineParams field of the CreateTableData object. It is also possible to specify default table engine params on schema creation: String url = "jdbc:h2:/data/sample;IFEXISTS=TRUE"; 2 Params from the schema are used when CREATE TABLE issued on this schema does not have its own engine params specified. TriggersThis database supports Java triggers that are called before or after a row is updated, inserted or deleted. Triggers can be used for complex consistency checks, or to update related data in the database. It is also possible to use triggers to simulate materialized views. For a complete sample application, see String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 24. A Java trigger must implement the interface String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 25. The trigger class must be available in the classpath of the database engine (when using the server mode, it must be in the classpath of the server). String url = "jdbc:h2:/data/sample;IFEXISTS=TRUE"; 3 The connection can be used to query or update data in other tables. The trigger then needs to be defined in the database: String url = "jdbc:h2:/data/sample;IFEXISTS=TRUE"; 4 The trigger can be used to veto a change by throwing a String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 26. As an alternative to implementing the String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 27 interface, an application can extend the abstract class String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 28. This will allows to use the String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 29 interface within trigger implementations. In this case, only the String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 30 method needs to be implemented: String url = "jdbc:h2:/data/sample;IFEXISTS=TRUE"; 5 Compacting a DatabaseEmpty space in the database file re-used automatically. When closing the database, the database is automatically compacted for up to 200 milliseconds by default. To compact more, use the SQL statement SHUTDOWN COMPACT. However re-creating the database may further reduce the database size because this will re-build the indexes. Here is a sample function to do this: String url = "jdbc:h2:/data/sample;IFEXISTS=TRUE"; 6 See also the sample application String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 31. The commands String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 32 can be used as well to create a backup of a database and re-build the database from the script. Cache SettingsThe database keeps most frequently used data in the main memory. The amount of memory used for caching can be changed using the setting String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 33. This setting can be set in the database connection URL ( String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 34), or it can be changed at runtime using String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 35. The size of the cache, as represented by String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 33 is measured in KB, with each KB being 1024 bytes. This setting has no effect for in-memory databases. For persistent databases, the setting is stored in the database and re-used when the database is opened the next time. However, when opening an existing database, the cache size is set to at most half the amount of memory available for the virtual machine (Runtime.getRuntime().maxMemory()), even if the cache size setting stored in the database is larger; however the setting stored in the database is kept. Setting the cache size in the database URL or explicitly using String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 37 overrides this value (even if larger than the physical memory). To get the current used maximum cache size, use the query String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 38 An experimental scan-resistant cache algorithm "Two Queue" (2Q) is available. To enable it, append String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 39 to the database URL. The cache might not actually improve performance. If you plan to use it, please run your own test cases first. Also included is an experimental second level soft reference cache. Rows in this cache are only garbage collected on low memory. By default the second level cache is disabled. To enable it, use the prefix String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 40. Example: String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 41. The cache might not actually improve performance. If you plan to use it, please run your own test cases first. To get information about page reads and writes, and the current caching algorithm in use, call String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 42. The number of pages read / written is listed. External authentication (Experimental)External authentication allows to optionally validate user credentials externally (JAAS,LDAP,custom classes). Is also possible to temporary assign roles to externally authenticated users. This feature is experimental and subject to change Master user cannot be externally authenticated To enable external authentication on a database execute statement String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 43. This setting in persisted on the database. To connect on a database by using external credentials client must append String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 44 to the database URL. String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 45 is the identifier of the authentication realm (see later). External authentication requires to send password to the server. For this reason is works only on local connection or remote over ssl By default external authentication is performed through JAAS login interface (configuration name is String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 46). To configure JAAS add argument String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 47 Here an example of JAAS login configuration file content: String url = "jdbc:h2:/data/sample;IFEXISTS=TRUE"; 7 Is it possible to specify custom authentication settings by using JVM argument String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET"; 48. Here an example of h2auth.xml file content: |