spring中jdbcTemplate归纳2
用语句创建器更新数据库
第一个回调接口是PerparedStatementCreator,实现这个接口来覆盖整个更新过程的语句创建任务。我们先看下源代码中的PerparedStatementCreator接口，然后实现我们自己的接
口。
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public interface PreparedStatementCreator {/** * Create a statement in this connection. Allows implementations to use * PreparedStatements. The JdbcTemplate will close the created statement. * @param con Connection to use to create statement * @return a prepared statement * @throws SQLException there is no need to catch SQLExceptions * that may be thrown in the implementation of this method. * The JdbcTemplate class will handle them. */PreparedStatement createPreparedStatement(Connection con) throws SQLException;}?
源代码中只定义了一个方法createPreparedStatement，在connection连接的基础上创建statement，使我们可以使用PreparedStatements实例，创建完，jdbcTemplate会自动关闭
Connection连接，下面我们实现这个接口。
public class InsertPreparedStatementCreator implements PreparedStatementCreator{private User user;public InsertPreparedStatementCreator(User user){this.user = user;}public PreparedStatement createPreparedStatement(Connection conn)throws Exception{String sql = "insert into user value(?,?);PreparedStatement ps = conn.prepareStatement(sql);ps.setString(user.getUsername());ps.setString(user.getPassword());return ps;}}?
我们实现后，只需要创建PreparedStatement ，并且把参数绑定上去，然后返回，在处理过程中不需要进行异常处理，因为方法签名中已经给我们抛出了异常，也不用关闭数据库
连接或者打开数据库连接，这些spring都给我们做好了，然后调用jdbcTemplate中的update方法即可。
jdbcTemplate.update(new InsertPreparedStatementCreator(user));
当然也可以用另一种方式。
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jdbcTemplate.update(new PreparedStatementCreator(){String sql = "insert into user value(?,?);PreparedStatement ps = conn.prepareStatement(sql);ps.setString(user.getUsername());ps.setString(user.getPassword());return ps;});?
这里其实是个回调函数，在设计模式中是模板方法模式。
模板方法使用继承机制实现。在父类中定义不变的，然后把变化的定义成一个抽象方法，供子类实现。因此模板方法的 父类一般也是一个抽象类。
在jdbcTemplate中的执行：
我们的update方法调用了jdbcTemplate方法中的
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public int update(PreparedStatementCreator psc) throws DataAccessException {return update(psc, (PreparedStatementSetter) null);}?
接着调用
protected int update(final PreparedStatementCreator psc, final PreparedStatementSetter pss)throws DataAccessException {if (logger.isDebugEnabled()) {String sql = getSql(psc);logger.debug("Executing SQL update" + (sql != null ? " [" + sql + "]" : ""));}Integer result = (Integer) execute(psc, new PreparedStatementCallback() {public Object doInPreparedStatement(PreparedStatement ps) throws SQLException {try {if (pss != null) {pss.setValues(ps);}int rows = ps.executeUpdate();if (logger.isDebugEnabled()) {logger.debug("SQL update affected " + rows + " rows");}return new Integer(rows);}finally {if (pss instanceof ParameterDisposer) {((ParameterDisposer) pss).cleanupParameters();}}}});return result.intValue();}?
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方法的关键：
int rows = ps.executeUpdate();
return new Integer(rows);
执行了preparedStatement.executeUpdate().然后返回执行后的值，和我们自己写是一样。在这里还是没有看到spring是如何打开数据库连接的。继续看，注意到一个方法的调用
Integer result = (Integer) execute(psc, new PreparedStatementCallback() 。。。。。。
我们找到这个方法
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public Object execute(PreparedStatementCreator psc, PreparedStatementCallback action)throws DataAccessException {Assert.notNull(psc, "PreparedStatementCreator must not be null");Assert.notNull(action, "Callback object must not be null");Connection con = DataSourceUtils.getConnection(getDataSource());PreparedStatement ps = null;try {Connection conToUse = con;if (this.nativeJdbcExtractor != null &&this.nativeJdbcExtractor.isNativeConnectionNecessaryForNativePreparedStatements()) {conToUse = this.nativeJdbcExtractor.getNativeConnection(con);}ps = psc.createPreparedStatement(conToUse);applyStatementSettings(ps);PreparedStatement psToUse = ps;if (this.nativeJdbcExtractor != null) {psToUse = this.nativeJdbcExtractor.getNativePreparedStatement(ps);}Object result = action.doInPreparedStatement(psToUse);SQLWarning warning = ps.getWarnings();throwExceptionOnWarningIfNotIgnoringWarnings(warning);return result;}catch (SQLException ex) {// Release Connection early, to avoid potential connection pool deadlock// in the case when the exception translator hasn't been initialized yet.if (psc instanceof ParameterDisposer) {((ParameterDisposer) psc).cleanupParameters();}String sql = getSql(psc);psc = null;JdbcUtils.closeStatement(ps);ps = null;DataSourceUtils.releaseConnection(con, getDataSource());con = null;throw getExceptionTranslator().translate("PreparedStatementCallback", sql, ex);}finally {if (psc instanceof ParameterDisposer) {((ParameterDisposer) psc).cleanupParameters();}JdbcUtils.closeStatement(ps);DataSourceUtils.releaseConnection(con, getDataSource());}}?
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里面有这么一句Connection con = DataSourceUtils.getConnection(getDataSource());
这里获取数据库数据库连接。我们看看他是怎么获取的数据库连接。
进入DataSourceUtils这个类中
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public static Connection getConnection(DataSource dataSource) throws CannotGetJdbcConnectionException {try {return doGetConnection(dataSource);}catch (SQLException ex) {throw new CannotGetJdbcConnectionException("Could not get JDBC Connection", ex);}}public static Connection doGetConnection(DataSource dataSource) throws SQLException {Assert.notNull(dataSource, "No DataSource specified");ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);if (conHolder != null && (conHolder.hasConnection() || conHolder.isSynchronizedWithTransaction())) {conHolder.requested();if (!conHolder.hasConnection()) {logger.debug("Fetching resumed JDBC Connection from DataSource");conHolder.setConnection(dataSource.getConnection());}return conHolder.getConnection();}// Else we either got no holder or an empty thread-bound holder here.logger.debug("Fetching JDBC Connection from DataSource");Connection con = dataSource.getConnection();if (TransactionSynchronizationManager.isSynchronizationActive()) {logger.debug("Registering transaction synchronization for JDBC Connection");// Use same Connection for further JDBC actions within the transaction.// Thread-bound object will get removed by synchronization at transaction completion.ConnectionHolder holderToUse = conHolder;if (holderToUse == null) {holderToUse = new ConnectionHolder(con);}else {holderToUse.setConnection(con);}holderToUse.requested();TransactionSynchronizationManager.registerSynchronization(new ConnectionSynchronization(holderToUse, dataSource));holderToUse.setSynchronizedWithTransaction(true);if (holderToUse != conHolder) {TransactionSynchronizationManager.bindResource(dataSource, holderToUse);}}return con;}?
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spring会先去ConnectionHolder 中获取Connection，如果有直接从ConnectionHolder 获取conennction. conHolder.getConnection();如果没有则conHolder.setConnection
(dataSource.getConnection());，然后继续conennction. conHolder.getConnection()，说白了Connection是从ConnectionHolder 中获取的。
我们继续往深处看看，ConnectionHolder 是怎么把Connection给我们的。我们进入ConnectionHolder这个类中
public class ConnectionHolder extends ResourceHolderSupport,ConnectionHolder继承自抽象类 ResourceHolderSupport，我们暂时先不讨论这个类，主要看
ConnectionHolder.
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public Connection getConnection() {Assert.notNull(this.connectionHandle, "Active Connection is required");if (this.currentConnection == null) {this.currentConnection = this.connectionHandle.getConnection();}return this.currentConnection;}?
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原来ConnectionHolder也不是现实取得Connection真正的类，它是从connectionHandle中获取的，
我们在进入ConnectionHandle这个处理类中，
public interface ConnectionHandle {/** * Fetch the JDBC Connection that this handle refers to. */Connection getConnection();/** * Release the JDBC Connection that this handle refers to. * @param con the JDBC Connection to release */void releaseConnection(Connection con);}?
发现什么也没有，只是一个接口，定义了一个标准。既然是一个接口，就一定有实现类，我们继续往下找。
public class SimpleConnectionHandle implements ConnectionHandle 仅有一个实现类，还是一个simple的，晕吧，spring文档竟然说是一个简单的实现。那复杂的实现在哪呢
？找了半天就找到这个一个简单的实现，有点郁闷。看看在说。
public class SimpleConnectionHandle implements ConnectionHandle {private final Connection connection;/** * Create a new SimpleConnectionHandle for the given Connection. * @param connection the JDBC Connection */public SimpleConnectionHandle(Connection connection) {Assert.notNull(connection, "Connection must not be null");this.connection = connection;}/** * Return the specified Connection as-is. */public Connection getConnection() {return connection;}/** * This implementation is empty, as we're using a standard * Connection handle that does not have to be released. */public void releaseConnection(Connection con) {}public String toString() {return "SimpleConnectionHandle: " + this.connection;}}?
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这里也没有我们想要的connection连接，只是一个保存connnection的地方，我们要的是DriverManager.getConnection()的地方，在哪呢？继续找吧！返回到抽象类
DataSourceUtils中，继续看。Connection con = dataSource.getConnection();看到了吧，其实spring也没有提供如何获取，是在sun的包中实现了DriverManager.getConnection
spring给我们提供了多种获取connnection的途径，还有
ConnectionHolder holderToUse = conHolder; if (holderToUse == null) { holderToUse = new ConnectionHolder(con); } public ConnectionHolder(Connection connection) { this.connectionHandle = new SimpleConnectionHandle(connection); }?
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???在我们第一次连接后，以后就吧connnection保存了，下次用的时候就直接存spring中拿就OK了。返回到我们的jdbcTemplate继续看我们刚才的代码,注意如下代码。
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Connection conToUse = con; if (this.nativeJdbcExtractor != null && this.nativeJdbcExtractor.isNativeConnectionNecessaryForNativePreparedStatements()) { conToUse = this.nativeJdbcExtractor.getNativeConnection(con); } ps = psc.createPreparedStatement(conToUse);?
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?psc.createPreparedStatement(conToUse);使用的是conToUse的Connection,而这个Connection是通过this.nativeJdbcExtractor.getNativeConnection获取的，我们看
看.nativeJdbcExtractor这个东东。
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public interface NativeJdbcExtractor {boolean isNativeConnectionNecessaryForNativeStatements();boolean isNativeConnectionNecessaryForNativePreparedStatements();boolean isNativeConnectionNecessaryForNativeCallableStatements();Connection getNativeConnection(Connection con) throws SQLException;Connection getNativeConnectionFromStatement(Statement stmt) throws SQLException;Statement getNativeStatement(Statement stmt) throws SQLException;PreparedStatement getNativePreparedStatement(PreparedStatement ps) throws SQLException;CallableStatement getNativeCallableStatement(CallableStatement cs) throws SQLException;ResultSet getNativeResultSet(ResultSet rs) throws SQLException;}?
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NativeJdbcExtractor 是一个接口。有getNativeConnection获取Connection的方法。我们继续看看NativeJdbcExtractor 接口的实现类。就恍然大悟了。
C3P0NativeJdbcExtractor, CommonsDbcpNativeJdbcExtractor, JBossNativeJdbcExtractor, NativeJdbcExtractorAdapter, SimpleNativeJdbcExtractor,
WebLogicNativeJdbcExtractor, WebSphereNativeJdbcExtractor, XAPoolNativeJdbcExtractor有这么多类实现了NativeJdbcExtractor 这个接口，名名字不难看出胡，有通过
jboss获取的,有本地jdbc（NativeJdbcExtractorAdapter）获取的，有weblogic获取的，有websphere获取的，基本上我们常见的服务器都支持。在这里。
我们看一个实现类NativeJdbcExtractorAdapter,
public Connection getNativeConnection(Connection con) throws SQLException {if (con == null) {return null;}Connection targetCon = DataSourceUtils.getTargetConnection(con);Connection nativeCon = doGetNativeConnection(targetCon);if (nativeCon == targetCon) {// We haven't received a different Connection, so we'll assume that there's// some additional proxying going on. Let's check whether we get something// different back from the DatabaseMetaData.getConnection() call.DatabaseMetaData metaData = targetCon.getMetaData();// The following check is only really there for mock Connections// which might not carry a DatabaseMetaData instance.if (metaData != null) {Connection metaCon = metaData.getConnection();if (metaCon != targetCon) {// We've received a different Connection there:// Let's retry the native extraction process with it.nativeCon = doGetNativeConnection(metaCon);}}}return nativeCon;}?
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通过这里DatabaseMetaData metaData = targetCon.getMetaData();获取connection，就看到这里，关于更详细的介绍且听下回分解。
2010年9月7日22:02:47