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());
}