关闭数据连接时, 一定也要关闭Statement和ResultSet, 不然在并发量较大的时候可能导致内存泄漏. (如果是用tomcat自带的数据源实现, 则可以通过添加interceptor实现自动关闭statement.)
但是, 关闭Statement和ResultSet是乏味的工作.
例如下面的代码:
@Test
public void testConnection() throw Exception{
Connection conn = null;
PreparedStatement ps = null;
ResultSet rs = null;
try{
conn = DsUtil.getConnection();
ps = conn.prepareStatement("select now()");
rs = ps.executeQuery();
//do something
}finally {
//DsUtil是一个工具类, 就不贴代码了, 你懂的.
DsUtil.close(rs);
DsUtil.close(ps);
DsUtil.close(conn);
}
}
上面的代码只有一个Statement和一个ResultSet, 关闭一下也不算太麻烦, 所以你可能觉得笔者没有必要写这篇文章.
但是, 如果有多个Statement和ResultSet呢?
考虑如下代码:
@Test
public void testConnection() throws Exception {
Connection conn = null;
PreparedStatement psFoo = null;
ResultSet rsFoo = null;
PreparedStatement psBar = null;
ResultSet rsBar = null;
try {
conn = DsUtil.getConnection();
psFoo = conn.prepareStatement("....");
rsFoo = psFoo.executeQuery();
psBar = conn.prepareStatement("....");
rsBar = psBar.executeQuery();
//do something
} finally {
DsUtil.close(rsFoo);
DsUtil.close(psFoo);
DsUtil.close(rsBar);
DsUtil.close(psBar);
DsUtil.close(conn);
}
}
上面的代码有两个statement和result, 关起来就不那么令人愉快了.
大多数程序员肯定是不喜欢关闭多个statement和result的. 我见到过偷懒的程序用这种写法来规避关闭多个statement的问题.
@Test
public void testConnectionNotGood() throws Exception {
Connection conn = null;
PreparedStatement psFoo = null;
ResultSet rsFoo = null;
try {
conn = DsUtil.getConnection();
psFoo = conn.prepareStatement("....");
rsFoo = psFoo.executeQuery();
//do something
psFoo = conn.prepareStatement("....");
rsFoo = psFoo.executeQuery();
//do something
} finally {
DsUtil.close(rsFoo);
DsUtil.close(psFoo);
DsUtil.close(conn);
}
}
上面这断代码的不妥之处, 是Statement和ResultSet被重用了. 实际上创建了两个Statement和Result, 但最后只关闭了一个. 这样显然是不对的, 属于鸵鸟政策, 没解决实际问题.
那么, 有没有办法实现一个自定义的Connection, 使得程序员不需要手动关闭Statement和Result, 并支持Statement/PrepareStatement/CallableStatement呢?
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简单地说:
我们希望connection持有statement的软引用, 而statement又持有resultset的软引用, 并分别重写connection和statement的close方法, 在关闭之前先关闭软引用中的对象.
详细地说:
1. 创建一个ResultSetStatementAwareConnection.
该自定义Connection会记住所有的Statement/PreparedStatement/CallableStatement实例.
2. 创建一个ResultSetAwareStatement, 记住所有Statement.
3. 创建一个ResultSetAwarePreparedStatement, 记住所有的PreparedStatement.
4. 创建一个ResultSetAwareCallableStatement, 记住所有的CallableStatement.
先说ResultSetStatementAwareConnection的实现.
class ResultSetStatementAwareConnection implements Connection {
private Map> openStatements = new ConcurrentHashMap>(3);
protected Connection underlyingConnection;
public ResultSetStatementAwareConnection(Connection conn) {
this.underlyingConnection = conn;
}
private void addToOpenStatement(Statement statement) {
openStatements.put(DsUtil.getIdentityHexString(statement), new SoftReference(statement));
}
public void close() throws SQLException {
try {
closeOpenStatements();
} finally {
closeUnderlyingConnection();
}
}
private void closeOpenStatements() {
for (Map.Entry> entry : openStatements.entrySet()) {
DsUtil.close(entry.getValue().get());
}
openStatements.clear();
openStatements = null;
}
protected void closeUnderlyingConnection() {
DsUtil.close(underlyingConnection);
underlyingConnection = null;
}
@Override
public Statement createStatement() throws SQLException {
ResultSetAwareStatement statement = ResultSetAwareStatement.decorate(underlyingConnection.createStatement());
statement.setConnection(this);
addToOpenStatement(statement);
return statement;
}
@Override
public PreparedStatement prepareStatement(String sql) throws SQLException {
ResultSetAwarePreparedStatement statement = ResultSetAwarePreparedStatement.decorate(underlyingConnection.prepareStatement(sql));
statement.setConnection(this);
addToOpenStatement(statement);
return statement;
}
@Override
public CallableStatement prepareCall(String sql) throws SQLException {
ResultSetAwareCallableStatement statement = ResultSetAwareCallableStatement.decorate(underlyingConnection.prepareCall(sql));
statement.setConnection(this);
addToOpenStatement(statement);
return statement;
}
@Override
public String getCatalog() throws SQLException {
return underlyingConnection.getCatalog();
}
//更多代码见附件
}
通过openStatements持有所有statement的软引用, 并且close方法中会先调用closeOpenStatements把软引用持有的statement全部关闭, 然后再通过closeUnderlyingConnection去真正关闭连接.
到现在为止, 就只需要关闭数据库连接, 不需要显式关闭statement了.
类似地, statement也可以通过这种模式来关闭resultset. 以PreparedStatement为例.
class ResultSetAwarePreparedStatement implements PreparedStatement {
private Map> openResultSets = new ConcurrentHashMap>(3);
private ResultSetStatementAwareConnection connection;
private PreparedStatement underlyingPreparedStatement;
static ResultSetAwarePreparedStatement decorate(PreparedStatement statement) {
ResultSetAwarePreparedStatement instance = new ResultSetAwarePreparedStatement();
instance.underlyingPreparedStatement = statement;
return instance;
}
private void addToOpenResultSet(ResultSet resultSet) {
openResultSets.put(DsUtil.getIdentityHexString(resultSet), new SoftReference(resultSet));
}
public void close() throws SQLException {
try {
closeOpenResultSets();
} finally {
closeUnderlyingStatement();
}
}
private void closeOpenResultSets() {
for (Map.Entry> entry : openResultSets.entrySet()) {
DsUtil.close(entry.getValue().get());
}
openResultSets.clear();
openResultSets = null;
}
private void closeUnderlyingStatement() {
DsUtil.close(underlyingPreparedStatement);
connection = null;
}
//更多代码见附件
}
通过openResultSets持有resultset的软引用, 并且close方法中会先调用closeOpenResultSets把软引用持有的resultset全部关闭, 然后再通过closeUnderlyingStatement去真正关闭statement.
到这里, 所有的事情就完成了. 下面举个栗子.
@Test
public void testConnectionNotGood() throws Exception {
Connection conn = null;
try {
conn = new ResultSetStatementAwareConnection(DsUtil.getConnection());
PreparedStatement psFoo = conn.prepareStatement("....");
ResultSet rsFoo = psFoo.executeQuery();
//do something
psFoo = conn.prepareStatement("....");
rsFoo = psFoo.executeQuery();
//do something
psFoo = conn.prepareStatement("....");
rsFoo = psFoo.executeQuery();
//do something
} finally {
DsUtil.close(conn);
}
}
我们看到PreparedStatement和ResultSet一共使用了三次, 创建了三个PreparedStatement和三个ResultSet. 但是在finally块中只显式关闭了Connection, 并没有显式关闭PreparedStatement和ResultSet.
不过放心, 虽然没有显式关闭, 但其实三个PrepareStatement和ResultSet都会被自动关闭.
本文完.
(在附件中, DsUtill并没有提供getConnection方法, 需要自行编写.)