openGauss-connector-jdbc/docs/documentation/94/connect.md

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default_docs	Connecting to the Database	Chapter 3. Initializing the Driver	media	Loading the Driver	load.html	Chapter 4. Using SSL	ssl.html

With JDBC, a database is represented by a URL (Uniform Resource Locator). With PostgreSQL™, this takes one of the following forms:

• jdbc:postgresql:database
• jdbc:postgresql:/
• jdbc:postgresql://host/database
• jdbc:postgresql://host/
• jdbc:postgresql://host:port/database
• jdbc:postgresql://host:port/

The parameters have the following meanings:

• host

  The host name of the server. Defaults to localhost. To specify an IPv6 address your must enclose the host parameter with square brackets, for example:

  jdbc:postgresql://[::1]:5740/accounting

• port

  The port number the server is listening on. Defaults to the PostgreSQL™ standard port number (5432).

• database

  The database name. The default is to connect to a database with the same name as the user name.

To connect, you need to get a Connection instance from JDBC. To do this, you use the DriverManager.getConnection() method:

Connection db = DriverManager.getConnection(url, username, password);

Connection Parameters

In addition to the standard connection parameters the driver supports a number of additional properties which can be used to specify additional driver behaviour specific to PostgreSQL™. These properties may be specified in either the connection URL or an additional Properties object parameter to DriverManager.getConnection. The following examples illustrate the use of both methods to establish a SSL connection.

String url = "jdbc:postgresql://localhost/test";
Properties props = new Properties();
props.setProperty("user","fred");
props.setProperty("password","secret");
props.setProperty("ssl","true");
Connection conn = DriverManager.getConnection(url, props);

String url = "jdbc:postgresql://localhost/test?user=fred&password=secret&ssl=true";
Connection conn = DriverManager.getConnection(url);

• user = String

  The database user on whose behalf the connection is being made.

• password = String

  The database user's password.

• ssl

  Connect using SSL. The driver must have been compiled with SSL support. This property does not need a value associated with it. The mere presence of it specifies a SSL connection. However, for compatibility with future versions, the value "true" is preferred. For more information see Chapter 4, Using SSL.

• sslfactory = String

  The provided value is a class name to use as the SSLSocketFactory when establishing a SSL connection. For more information see the section called “Custom SSLSocketFactory”.

• sslfactoryarg = String

  This value is an optional argument to the constructor of the sslfactory class provided above. For more information see the section called “Custom SSLSocketFactory”.

• socketFactory = String Specify a socket factory for socket creation.

• socketFactoryArg = String Argument forwarded to constructor of SocketFactory class

• compatible = String

  Act like an older version of the driver to retain compatibility with older applications. At the moment this controls two driver behaviours: the handling of binary data fields, and the handling of parameters set via setString().

  Older versions of the driver used this property to also control the protocol used to connect to the backend. This is now controlled by the protocolVersion property.

  Information on binary data handling is detailed in Chapter 7, Storing Binary Data. To force the use of Large Objects set the compatible property to 7.1.

  When compatible is set to 7.4 or below, the default for the stringtype parameter is changed to unspecified.

• sendBufferSize = int

  Sets SO_SNDBUF on the connection stream

• recvBufferSize = int

  Sets SO_RCVBUF on the connection stream

• receiveBufferSize = int

  Sets SO_RCVBUF on the connection stream

• protocolVersion = String

  The driver supports both the V2 and V3 frontend/backend protocols. The V3 protocol was introduced in 7.4 and the driver will by default try to connect using the V3 protocol, if that fails it will fall back to the V2 protocol. If the protocolVersion property is specified, the driver will try only the specified protocol (which should be either "2" or "3"). Setting protocolVersion to "2" may be used to avoid the failed attempt to use the V3 protocol when connecting to a version 7.3 or earlier server, or to force the driver to use the V2 protocol despite connecting to a 7.4 or greater server.

• loglevel = int

  Set the amount of logging information printed to the DriverManager's current value for LogStream or LogWriter. It currently supports values of org.postgresql.Driver.DEBUG (2) and org.postgresql.Driver.INFO (1). INFO will log very little information while DEBUG will produce significant detail. This property is only really useful if you are a developer or are having problems with the driver.

• charSet = String

  The character set to use for data sent to the database or received from the database. This property is only relevant for server versions less than or equal to 7.2. The 7.3 release was the first with multibyte support compiled by default and the driver uses its character set translation facilities instead of trying to do it itself.

• allowEncodingChanges = boolean

  When using the V3 protocol the driver monitors changes in certain server configuration parameters that should not be touched by end users. The client_encoding setting is set by the driver and should not be altered. If the driver detects a change it will abort the connection. There is one legitimate exception to this behaviour though, using the COPY command on a file residing on the server's filesystem. The only means of specifying the encoding of this file is by altering the client_encoding setting. The JDBC team considers this a failing of the COPY command and hopes to provide an alternate means of specifying the encoding in the future, but for now there is this URL parameter. Enable this only if you need to override the client encoding when doing a copy.

• logUnclosedConnections = boolean

  Clients may leak Connection objects by failing to call its close() method. Eventually these objects will be garbage collected and the finalize() method will be called which will close the Connection if caller has neglected to do this himself. The usage of a finalizer is just a stopgap solution. To help developers detect and correct the source of these leaks the logUnclosedConnections URL parameter has been added. It captures a stacktrace at each Connection opening and if the finalize() method is reached without having been closed the stacktrace is printed to the log.

• binaryTransferEnable = String

  A comma separated list of types to enable binary transfer. Either OID numbers or names.

• binaryTransferDisable = String

  A comma separated list of types to disable binary transfer. Either OID numbers or names. Overrides values in the driver default set and values set with binaryTransferEnable.

• prepareThreshold = int

  Determine the number of PreparedStatement executions required before switching over to use server side prepared statements. The default is five, meaning start using server side prepared statements on the fifth execution of the same PreparedStatement object. More information on server side prepared statements is available in the section called “Server Prepared Statements”.

• preparedStatementCacheQueries = int

  Determine the number of queries that are cached in each connection. The default is 256, meaning if you use more than 256 different queries in prepareStatement() calls, the least recently used ones will be discarded. The cache allows application to benefit from “Server Prepared Statements” prepareThreshold even if the prepared statement is closed after each execution. The value of 0 disables the cache.

  Each connection has its own statement cache.

• preparedStatementCacheSizeMiB = int

  Determine the maximum size (in mebibytes) of the prepared queries cache (see preparedStatementCacheQueries). The default is 5, meaning if you happen to cache more than 5 MiB of queries the least recently used ones will be discarded. The main aim of this setting is to prevent OutOfMemoryError. The value of 0 disables the cache. If a query would consume more than a half of preparedStatementCacheSizeMiB, then it is discarded immediately.

• defaultRowFetchSize = int

  Determine the number of rows fetched in ResultSet by one fetch with trip to the database. Limiting the number of rows are fetch with each trip to the database allow avoids unnecessary memory consumption and as a consequence OutOfMemoryException The default is zero, meaning that in ResultSet will be fetch all rows at once. Negative number is not available.

• reWriteBatchedInserts = bool Enable optimization to rewrite and collapse compatible INSERT statements that are batched. If enabled, pgjdbc rewrites batch of insert into ... values(?, ?) into insert into ... values(?, ?), (?, ?), ... That reduces per-statement overhead. The drawback is if one of the statements fail, the whole batch fails. The default value is false. The option is available since 9.4.1208

• loginTimeout = int

  Specify how long to wait for establishment of a database connection. The timeout is specified in seconds.

• connectTimeout = int

  The timeout value used for socket connect operations. If connecting to the server takes longer than this value, the connection is broken. The timeout is specified in seconds and a value of zero means that it is disabled. The default value is 0 (unlimited) up to 9.4.1208, and 10 seconds since 9.4.1209

• cancelSignalTimeout = int (since 9.4.1209) Cancel command is sent out of band over its own connection, so cancel message can itself get stuck. This property controls "connect timeout" and "socket timeout" used for cancel commands. The timeout is specified in seconds. Default value is 10 seconds.

• socketTimeout = int

  The timeout value used for socket read operations. If reading from the server takes longer than this value, the connection is closed. This can be used as both a brute force global query timeout and a method of detecting network problems. The timeout is specified in seconds and a value of zero means that it is disabled.

• tcpKeepAlive = boolean

  Enable or disable TCP keep-alive probe. The default is false.

• unknownLength = int

  Certain postgresql types such as TEXT do not have a well defined length. When returning meta-data about these types through functions like ResultSetMetaData.getColumnDisplaySize and ResultSetMetaData.getPrecision we must provide a value and various client tools have different ideas about what they would like to see. This parameter specifies the length to return for types of unknown length.

• stringtype = String

  Specify the type to use when binding PreparedStatement parameters set via setString(). If stringtype is set to VARCHAR (the default), such parameters will be sent to the server as varchar parameters. If stringtype is set to unspecified, parameters will be sent to the server as untyped values, and the server will attempt to infer an appropriate type. This is useful if you have an existing application that uses setString() to set parameters that are actually some other type, such as integers, and you are unable to change the application to use an appropriate method such as setInt().

• kerberosServerName = String

  The Kerberos service name to use when authenticating with GSSAPI. This is equivalent to libpq's PGKRBSRVNAME environment variable and defaults to "postgres".

• jaasApplicationName = String

  Specifies the name of the JAAS system or application login configuration.

• ApplicationName = String

  Specifies the name of the application that is using the connection. This allows a database administrator to see what applications are connected to the server and what resources they are using through views like pg_stat_activity.

• gsslib = String

  Force either SSPI (Windows transparent single-sign-on) or GSSAPI (Kerberos, via JSSE) to be used when the server requests Kerberos or SSPI authentication. Permissible values are auto (default, see below), sspi (force SSPI) or gssapi (force GSSAPI-JSSE).

  If this parameter is auto, SSPI is attempted if the server requests SSPI authentication, the JDBC client is running on Windows, and the Waffle libraries required for SSPI are on the CLASSPATH. Otherwise Kerberos/GSSAPI via JSSE is used. Note that this behaviour does not exactly match that of libpq, which uses Windows' SSPI libraries for Kerberos (GSSAPI) requests by default when on Windows.

  gssapi mode forces JSSE's GSSAPI to be used even if SSPI is available, matching the pre-9.4 behaviour.

  On non-Windows platforms or where SSPI is unavailable, forcing sspi mode will fail with a PSQLException.

  Since: 9.4

• sspiServiceClass = String

  Specifies the name of the Windows SSPI service class that forms the service class part of the SPN. The default, POSTGRES, is almost always correct.

  See: SSPI authentication (Pg docs) Service Principal Names (MSDN), DsMakeSpn (MSDN) Configuring SSPI (Pg wiki).

  This parameter is ignored on non-Windows platforms.

• useSpnego = boolean

  Use SPNEGO in SSPI authentication requests

• readOnly = boolean

  Put the connection in read-only mode

• disableColumnSanitiser = boolean

  Enable optimization that disables column name sanitiser.

• assumeMinServerVersion = String

  Assume that the server is at least the given version, thus enabling to some optimization at connection time instead of trying to be version blind.

• currentSchema = String

  Specify the schema to be set in the search-path. This schema will be used to resolve unqualified object names used in statements over this connection.

• targetServerType

  Allows opening connections to only servers with required state, the allowed values are any, master, slave and preferSlave. The master/slave distinction is currently done by observing if the server allows writes. The value preferSlave tries to connect to slaves if any are available, otherwise allows falls back to connecting also to master.

• hostRecheckSeconds = int

  Controls how long in seconds the knowledge about a host state is cached in JVM wide global cache. The default value is 10 seconds.

• loadBalanceHosts = boolean

  In default mode (disabled) hosts are connected in the given order. If enabled hosts are chosen randomly from the set of suitable candidates.

  Connection Fail-over

  To support simple connection fail-over it is possible to define multiple endpoints (host and port pairs) in the connection url separated by commas. The driver will try to once connect to each of them in order until the connection succeeds. If none succeed, a normal connection exception is thrown.

  The syntax for the connection url is:

  jdbc:postgresql://host1:port1,host2:port2/database

  The simple connection fail-over is useful when running against a high availability postgres installation that has identical data on each node. For example streaming replication postgres or postgres-xc cluster.

  For example an application can create two connection pools. One data source is for writes, another for reads. The write pool limits connections only to master node:

  jdbc:postgresql://node1,node2,node3/accounting?targetServerType=master . And read pool balances connections between slaves nodes, but allows connections also to master if no slaves are available:

  jdbc:postgresql://node1,node2,node3/accounting?targetServerType=preferSlave&loadBalanceHosts=true