JDBC：Connection.close()

https://www.2cto.com/database/201501/369246.html

Connection对象在执行close() 方法之后，并不是直接把Connection对象设置为null （Connection对象有一个方法， isClosed() 判断Connection是否被关闭，如果直接设置为null，再执行isClosed() 就会报空指针异常）

close()源码：

public void close() throws SQLException {
    synchronized (getConnectionMutex()) {
        if (this.connectionLifecycleInterceptors != null) {
            new IterateBlock<Extension>(this.connectionLifecycleInterceptors.iterator()) {
                @Override
                void forEach(Extension each) throws SQLException {
                    ((ConnectionLifecycleInterceptor) each).close();
                }
            }.doForAll();
        }

        realClose(true, true, false, null);
    }
}

测试代码：

DBUtil是自己写的一个工具类，有静态方法 ResultSet generalQuery()，静态属性conn, pst 

public class Test {
    public static void main(String[] args) {
        String sql = "select * from member where mId = ? and mPwd = ?";
        Object[] params = {"2015", "1233"};
        ResultSet rs = DBUtil.generalQuery(sql, params);int cnt = 0;
        try {
            if(rs != null) {
                rs.last();
                cnt = rs.getRow();
            }
            if(rs != null) rs.close();
            if(DBUtil.pst != null) DBUtil.pst.close();
            if(DBUtil.conn != null) DBUtil.conn.close();
            
            if(cnt != 0) {
                System.out.println("查询成功！");
            }else {
                System.out.println("查询失败");
            }
        }catch (SQLException e) {
            e.printStackTrace();
        }
        try {
            System.out.println(DBUtil.conn);  // com.mysql.jdbc.JDBC4Connection@619a5dff
            System.out.println(DBUtil.pst);   // java.sql.SQLException: No operations allowed after statement closed.
            System.out.println(rs); //com.mysql.jdbc.JDBC42ResultSet@23ab930d
            
            System.out.println(DBUtil.conn.isClosed());  //true
            System.out.println(DBUtil.pst.isClosed());  //true
            System.out.println(rs.isClosed()); //true
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}