spring jdbcTemplate源码剖析

　　本文浅析 spring jdbcTemplate 源码，主要是学习其设计精髓。模板模式、巧妙的回调

一、jdbcTemplate 类结构

①、JdbcOperations : 接口定义了方法，如

<T> T execute(StatementCallback<T> action) throws DataAccessException;

void execute(String sql) throws DataAccessException;

<T> T query(String sql, ResultSetExtractor<T> rse) throws DataAccessException;

。。。

②、JdbcAccessor ： 定义了数据源。

③、实现JdbcOperations接口定义的方法

二、jdbcTemplate 模板模式

1、我们看下execute(StatementCallback<T> action)实现 （核心）

public <T> T execute(StatementCallback<T> action) throws DataAccessException {
        Assert.notNull(action, "Callback object must not be null");

        Connection con = DataSourceUtils.getConnection(getDataSource());
        Statement stmt = null;
        try {
            Connection conToUse = con;
            if (this.nativeJdbcExtractor != null &&
                    this.nativeJdbcExtractor.isNativeConnectionNecessaryForNativeStatements()) {
                conToUse = this.nativeJdbcExtractor.getNativeConnection(con);
            }
            stmt = conToUse.createStatement();
            applyStatementSettings(stmt);
            Statement stmtToUse = stmt;
            if (this.nativeJdbcExtractor != null) {
                stmtToUse = this.nativeJdbcExtractor.getNativeStatement(stmt);
            }
            T result = action.doInStatement(stmtToUse); // 回调
            handleWarnings(stmt);
            return result;
        }
        catch (SQLException ex) {
            JdbcUtils.closeStatement(stmt);
            stmt = null;
            DataSourceUtils.releaseConnection(con, getDataSource());
            con = null;
            throw getExceptionTranslator().translate("StatementCallback", getSql(action), ex);
        }
        finally {
            JdbcUtils.closeStatement(stmt);
            DataSourceUtils.releaseConnection(con, getDataSource());
        }
    }

2、再看下 execute(String sql) 源码

public void execute(final String sql) throws DataAccessException {
        if (logger.isDebugEnabled()) {
            logger.debug("Executing SQL statement [" + sql + "]");
        }
        class ExecuteStatementCallback implements StatementCallback<Object>, SqlProvider {
            @Override
            public Object doInStatement(Statement stmt) throws SQLException {
                stmt.execute(sql); // JAVA  jdbc
                return null;
            }
            @Override
            public String getSql() {
                return sql;
            }
        }
        execute(new ExecuteStatementCallback()); // 调用上述T execute(StatementCallback<T> action) 
    }

由此，可以知道，我们平时常用的execute(final String sql) 方法，底层帮我们做了很多事情，如创建默认的ExecuteStatementCallback，采用回调的方式，由模板去执行sql，关闭链接

3、T query(final String sql, final ResultSetExtractor<T> rse) 源码实现

@Override
    public <T> T query(final String sql, final ResultSetExtractor<T> rse) throws DataAccessException {
        Assert.notNull(sql, "SQL must not be null");
        Assert.notNull(rse, "ResultSetExtractor must not be null");
        if (logger.isDebugEnabled()) {
            logger.debug("Executing SQL query [" + sql + "]");
        }
        class QueryStatementCallback implements StatementCallback<T>, SqlProvider {
            @Override
            public T doInStatement(Statement stmt) throws SQLException {
                ResultSet rs = null;
                try {
                    rs = stmt.executeQuery(sql);
                    ResultSet rsToUse = rs;
                    if (nativeJdbcExtractor != null) {
                        rsToUse = nativeJdbcExtractor.getNativeResultSet(rs);
                    }
                    return rse.extractData(rsToUse);
                }
                finally {
                    JdbcUtils.closeResultSet(rs);
                }
            }
            @Override
            public String getSql() {
                return sql;
            }
        }
        return execute(new QueryStatementCallback());
    }

4、update(final String sql) 源码

@Override
    public int update(final String sql) throws DataAccessException {
        Assert.notNull(sql, "SQL must not be null");
        if (logger.isDebugEnabled()) {
            logger.debug("Executing SQL update [" + sql + "]");
        }
        class UpdateStatementCallback implements StatementCallback<Integer>, SqlProvider {
            @Override
            public Integer doInStatement(Statement stmt) throws SQLException {
                int rows = stmt.executeUpdate(sql);
                if (logger.isDebugEnabled()) {
                    logger.debug("SQL update affected " + rows + " rows");
                }
                return rows;
            }
            @Override
            public String getSql() {
                return sql;
            }
        }
        return execute(new UpdateStatementCallback());
    }