Jdbc Url 设置allowMultiQueries为true和false时底层处理机制研究

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一个mysql jdbc待解之谜关于jdbc url参数 allowMultiQueries

如下的一个普通JDBC示例:

String user ="root"; String password = "root"; String url = "jdbc:mysql://localhost:3306";Connection conn = java.sql.DriverManager.getConnection(url , user, password); Statement stmt = conn.createStatement(); String sql = "select 'hello';select 'world'"; stmt.execute(sql);

执行时会报错:

com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'select 'world'' at line 1 at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)

若一个sql中通过分号分割(或包含)了多个独立sql的话,如:

select 'hello';select 'world'

默认就会报上述错误,当若显式设置allowMultiQueries为true的话,就可以正常执行不会报错.如下所示:

String url = "jdbc:mysql://localhost:3306?allowMultiQueries=true";

官方文档解释:

allowMultiQueries Allow the use of ';' to delimit multiple queries during one statement (true/false), defaults to 'false', and does not affect the addBatch() and executeBatch() methods, which instead rely on rewriteBatchStatements. Default: false Since version: 3.1.1

想要看看当该参数设置为true和false时,源码中到底哪里有不同.经过断点调试发现了在下面代码处会有不同的结果:

//所属类和方法:void java.net.SocketOutputStream.socketWrite(byte[] b, int off, int len) throws IOException socketWrite0(fd, b, off, len); bytesWritten = len;

当设置为true时,执行完socketWrite0方法后查询query log,会看到这样的输出:

23 Queryselect 'hello'; 23 Queryselect 'world'

当设置为false时,执行完socketWrite0后.查询query log没有任何输出.

但奇怪的是它们的参数都一样,为什么一个有输出一个就没有呢?如下所示:

设置为true参数信息

设置为false时的参数信息:

补充: 即使当allowMultiQueries为false时,服务端也收到了查询请求,只不过没有在query log中输出而已.

如下抓包工具(Wireshark)所示:

又经过进一步调试,发现如下几处代码可能比较关键,如下所示:

/** * * 所属: void com.mysql.jdbc.MysqlIO.proceedHandshakeWithPluggableAuthentication(String user, String * password, String database, Buffer challenge) throws SQLException */ // We allow the user to configure whether // or not they want to support multiple queries // (by default, this is disabled). if (this.connection.getAllowMultiQueries()) { //1701行 this.clientParam |= CLIENT_MULTI_STATEMENTS; }last_sent = new Buffer(packLength); last_sent.writeLong(this.clientParam); //1876行

执行完上述语句后,对应的query log为:

150507 21:23:16 19 Connectroot@localhost on

似乎是在这一交互过程中通知了服务器端客户端允许一次查询多条语句.通过抓包工具(wireshark)可以看到在创建连接过程中更多的交互信息,如下所示:

但在调试过程中又遇到了一个新问题,若在上述代码处加上了断点,就会出现如下异常:

Caused by: java.io.EOFException: Can not read response from server. Expected to read 4 bytes, read 0 bytes before connection was unexpectedly lost. at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:3161) at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:3615) ... 43 more

尚未找到该问题原因(如在何处设置的超时时间).但是实验发现,在创建连接过程中不能有debug调试(即不能有暂停),否则就会有fin包(不知这是Mysql的协议还是TCP的协议?).如下所示:

关于fin包的描述见wiki

但在成功建立连接后进行断点调试没有问题.

补充: 通过telnet来模拟上述现象.

# telnet 127.0.0.1 3306 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'. J 5.6.17 <4/-7j1S- � 18fwG:-nh=66mysql_native_passwordConnection closed by foreign host.

对应的抓包信息:

因未能及时输入密码,导致服务端发出FIN包中断了连接.

或者也可以通过如下Java代码来模拟此现象:

Socket socket = new Socket("127.0.0.1",3306); System.in.read();

此时看到的抓包信息如下所示:

补充:

1. 如何查看query log:

mysql> show variables like 'general_log%'; +------------------+--------------------------------------------------------------------+ | Variable_name | Value | +------------------+--------------------------------------------------------------------+ | general_log | OFF | | general_log_file | /opt/mysql/server-5.6/data/zhuguowei-Presario-CQ43-Notebook-PC.log | +------------------+--------------------------------------------------------------------+ mysql> set global general_log = 'on' ; #开启另一终端 tail -f /opt/mysql/server-5.6/data/zhuguowei-Presario-CQ43-Notebook-PC.log

2. 关于使用抓包工具(Wireshark)

参考文档:

http://www.maketecheasier.com/using-wireshark-ubuntu/

http://floss.zoomquiet.io/data/20120511105248/index.html

注意:

本地调试的话Interface(网卡)选择any(或lo), 过滤条件如下所示:

其他问题:

1. 为什么在mysql交互终端中执行的命令不能被Wireshark捕捉到?

需要显式指定-h参数如

mysql -u root -p -h 127.0.0.1

注意若为-h localhost的话,仍不能被Wireshark捕捉到,不知两者有什么区别?通过StackOverFlow中提问得到解答.

On Unix, MySQL programs treat the host name localhost specially, in a way that is likely different from what you expect compared to other network-based programs. For connections to localhost, MySQL programs attempt to connect to the local server by using a Unix socket file. This occurs even if a --port or -P option is given to specify a port number. To ensure that the client makes a TCP/IP connection to the local server, use --host or -h to specify a host name value of 127.0.0.1, or the IP address or name of the local server. You can also specify the connection protocol explicitly, even for localhost, by using the --protocol=TCP option.

摘自: https://dev.mysql.com/doc/refman/5.0/en/connecting.html

2. 在mysql交互终端中是否默认设置了allowMultiQueries=true, 能有办法将其改为false吗?

确实默认设置了allowMultiQueries=true, 如下所示: