JDBC操作MySQL(1)—PreparedStatement

时间:2021-5-25 作者:qvyue

JDBC作为JAVA访问数据库的一套规范与标准,统一了数据库操作的API,大大简化了程序开发工作。不过由于历史原因,MySQL对JDBC默认的实现与规范定义或者说其它数据库如Oracle并不一致,为了更完整记录这些差异,计划抽空写个系列,整理下这些可能会误解的常用功能。第一篇首先聊聊PreparedStatement。

MySQL JDBC PreparedStatement

Prepare SQL的产生原因与实现原理

数据库SQL执行过程包括以下阶段: 词法分析->语法分析->语义分析->执行计划优化->执行。【词法分析->语法分析】这两个阶段称之为硬解析。词法分析识别SQL中每个词,语法分析解析SQL语句是否符合(SQL92、99、方言等)语法,并得到一棵语法树。

其实基于SQL的架构设计,基本都有这样一个处理过程,TDDL、ShardingJDBC、MyCAT都如此,当然这些产品都相比于数据库,支持的关键词、语法都只是其子集。另外SQL解析器可基于Yacc、Lex、Antlr、Javacc等构建,当然如果对解析性能要更好要求,则需要进行一个纯手工编写的解析器,例如阿里的Druid中的SQL解析器,应用可基于Vistor模式进行使用。

Prepare SQL也叫预编译SQL、Prepared Statements或者Parameterized Statements,就是将这类SQL中的值用占位符?替代,可以视为将SQL语句模板化或者说参数化。预编译语句的优势在于归纳为:一次编译、多次运行,省去了解析优化等过程。

Prepare的出现就是为了优化硬解析的问题,Prepare在服务器端的执行过程如下:

  • 【Prepare】 接收客户端带?的SQL, 硬解析得到语法树(stmt->Lex), 缓存在线程所在的PS cache中。此cache是一个HASH MAP. Key为stmt->id. 然后返回客户端stmt->id等信息。
  • 【Execute】接收客户端stmt->id和参数等信息(客户端不需要再发SQL过来)。服务器根据stmt->id在PS cache中查找得到硬解析后的stmt, 并设置参数,就可以继续后面的优化和执行。
    Prepare在execute阶段可以节省硬解析的时间。因此prepare适用于频繁执行的SQL。

    JDBC操作MySQL(1)—PreparedStatement

Prepare的另一个作用是防止SQL注入,这个是纯客户端JDBC通过转义实现的。这也是一般更推荐使用PreparedStatement而不是Statement的主要理由。防SQL注入的具体实现可以参见MySQL驱动中com.mysql.jdbc.PreparedStatement.setString代码。

MySQL驱动中PrepareStament的实现逻辑

看完Prepare的功能原理后,我们看下JDBC操作MySQL时的PreparaStatement,
在com.mysql.jdbc.ConnectionImpl类中

public java.sql.PreparedStatement prepareStatement(String sql,
            int resultSetType, int resultSetConcurrency) throws SQLException {
        synchronized (getConnectionMutex()) {
            checkClosed();
    
            //
            // FIXME: Create warnings if can't create results of the given
            // type or concurrency
            //
            PreparedStatement pStmt = null;
            
            boolean canServerPrepare = true;
            
            String nativeSql = getProcessEscapeCodesForPrepStmts() ? nativeSQL(sql): sql;
            
            if (this.useServerPreparedStmts && getEmulateUnsupportedPstmts()) {
                canServerPrepare = canHandleAsServerPreparedStatement(nativeSql);
            }
            
            if (this.useServerPreparedStmts && canServerPrepare) {
                if (this.getCachePreparedStatements()) {
                    synchronized (this.serverSideStatementCache) {
                        pStmt = (com.mysql.jdbc.ServerPreparedStatement)this.serverSideStatementCache.remove(sql);
                        
                        if (pStmt != null) {
                            ((com.mysql.jdbc.ServerPreparedStatement)pStmt).setClosed(false);
                            pStmt.clearParameters();
                        }
    
                        if (pStmt == null) {
                            try {
                                pStmt = ServerPreparedStatement.getInstance(getLoadBalanceSafeProxy(), nativeSql,
                                        this.database, resultSetType, resultSetConcurrency);
                                if (sql.length() 

从以上源代码中,我们看到其实MySQL的Prepare竟然有两种,分为是客户端(JDBC4PreparedStatement)与服务器端(ServerPrepareStatement),根据应用连接参数设置(useServerPrepStmts),选择不同的PreparedStatement。另外还会根据缓存参数设置(cachePrepStmts),选择是否从缓存重获取解析对象,该缓存是针对连接的,这对于应用端使用连接池的场景是比较适用的。

不同参数对应的Prepare区别

分别设置不同参数,查看服务器端操作日志。

  1. 使用客户端PreparedStatement,不开启缓存
public static void selectWithClientPs(int count) throws SQLException{
        Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "root", "123456");
        
        long begin = System.currentTimeMillis();
        for(int i=0;i

MySQL服务器执行日志:

Time                 Id Command    Argument
181225 13:23:43     1 Connect   root@localhost on test
            1 Query /* mysql-connector-java-5.1.46 ( Revision: 9cc87a48e75c2d2e87c1a293b2862ce651cb256e ) */SELECT  @@session.auto_increment_increment AS auto_increment_increment, @@character_set_client AS character_set_client, @@character_set_connection AS character_set_connection, @@character_set_results AS character_set_results, @@character_set_server AS character_set_server, @@collation_server AS collation_server, @@init_connect AS init_connect, @@interactive_timeout AS interactive_timeout, @@license AS license, @@lower_case_table_names AS lower_case_table_names, @@max_allowed_packet AS max_allowed_packet, @@net_buffer_length AS net_buffer_length, @@net_write_timeout AS net_write_timeout, @@query_cache_size AS query_cache_size, @@query_cache_type AS query_cache_type, @@sql_mode AS sql_mode, @@system_time_zone AS system_time_zone, @@time_zone AS time_zone, @@tx_isolation AS transaction_isolation, @@wait_timeout AS wait_timeout
            1 Query SET NAMES latin1
            1 Query SET character_set_results = NULL
            1 Query SET autocommit=1
            1 Query select * from test where id= 0
            1 Query select * from test where id= 1
            1 Query select * from test where id= 2
            1 Quit
  1. 使用客户端PreparedStatement,开启缓存
    public static void selectWithClientPsAndCache(int count) throws SQLException{
        Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/test?cachePrepStmts=true", "root", "123456");

        long begin = System.currentTimeMillis();
        for(int i=0;i

MySQL服务器执行日志:

            2 Connect   root@localhost on test
            2 Query /* mysql-connector-java-5.1.46 ( Revision: 9cc87a48e75c2d2e87c1a293b2862ce651cb256e ) */SELECT  @@session.auto_increment_increment AS auto_increment_increment, @@character_set_client AS character_set_client, @@character_set_connection AS character_set_connection, @@character_set_results AS character_set_results, @@character_set_server AS character_set_server, @@collation_server AS collation_server, @@init_connect AS init_connect, @@interactive_timeout AS interactive_timeout, @@license AS license, @@lower_case_table_names AS lower_case_table_names, @@max_allowed_packet AS max_allowed_packet, @@net_buffer_length AS net_buffer_length, @@net_write_timeout AS net_write_timeout, @@query_cache_size AS query_cache_size, @@query_cache_type AS query_cache_type, @@sql_mode AS sql_mode, @@system_time_zone AS system_time_zone, @@time_zone AS time_zone, @@tx_isolation AS transaction_isolation, @@wait_timeout AS wait_timeout
            2 Query SET NAMES latin1
            2 Query SET character_set_results = NULL
            2 Query SET autocommit=1
            2 Query select * from test where id= 0
            2 Query select * from test where id= 1
            2 Query select * from test where id= 2
            2 Quit  
  1. 使用服务器端PreparedStatement,不开启缓存
public static void selectWithServerPs(int count) throws SQLException{
        Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/test?useServerPrepStmts=true", "root", "123456");
        
        long begin = System.currentTimeMillis();
        for(int i=0;i

MySQL服务器执行日志:

            3 Connect   root@localhost on test
            3 Query /* mysql-connector-java-5.1.46 ( Revision: 9cc87a48e75c2d2e87c1a293b2862ce651cb256e ) */SELECT  @@session.auto_increment_increment AS auto_increment_increment, @@character_set_client AS character_set_client, @@character_set_connection AS character_set_connection, @@character_set_results AS character_set_results, @@character_set_server AS character_set_server, @@collation_server AS collation_server, @@init_connect AS init_connect, @@interactive_timeout AS interactive_timeout, @@license AS license, @@lower_case_table_names AS lower_case_table_names, @@max_allowed_packet AS max_allowed_packet, @@net_buffer_length AS net_buffer_length, @@net_write_timeout AS net_write_timeout, @@query_cache_size AS query_cache_size, @@query_cache_type AS query_cache_type, @@sql_mode AS sql_mode, @@system_time_zone AS system_time_zone, @@time_zone AS time_zone, @@tx_isolation AS transaction_isolation, @@wait_timeout AS wait_timeout
            3 Query SET NAMES latin1
            3 Query SET character_set_results = NULL
            3 Query SET autocommit=1
            3 Prepare   select * from test where id= ?
            3 Execute   select * from test where id= 0
            3 Close stmt    
            3 Prepare   select * from test where id= ?
            3 Execute   select * from test where id= 1
            3 Close stmt    
            3 Prepare   select * from test where id= ?
            3 Execute   select * from test where id= 2
            3 Close stmt    
            3 Quit  
  1. 使用服务器端PreparedStatement,开启缓存
   public static void selectWithServerPsAndCache(int count) throws SQLException{
       Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/test?useServerPrepStmts=true&cachePrepStmts=true", "root", "123456");
       
       long begin = System.currentTimeMillis();
       for(int i=0;i

MySQL服务器执行日志:

            4 Connect   root@localhost on test
            4 Query /* mysql-connector-java-5.1.46 ( Revision: 9cc87a48e75c2d2e87c1a293b2862ce651cb256e ) */SELECT  @@session.auto_increment_increment AS auto_increment_increment, @@character_set_client AS character_set_client, @@character_set_connection AS character_set_connection, @@character_set_results AS character_set_results, @@character_set_server AS character_set_server, @@collation_server AS collation_server, @@init_connect AS init_connect, @@interactive_timeout AS interactive_timeout, @@license AS license, @@lower_case_table_names AS lower_case_table_names, @@max_allowed_packet AS max_allowed_packet, @@net_buffer_length AS net_buffer_length, @@net_write_timeout AS net_write_timeout, @@query_cache_size AS query_cache_size, @@query_cache_type AS query_cache_type, @@sql_mode AS sql_mode, @@system_time_zone AS system_time_zone, @@time_zone AS time_zone, @@tx_isolation AS transaction_isolation, @@wait_timeout AS wait_timeout
            4 Query SET NAMES latin1
            4 Query SET character_set_results = NULL
            4 Query SET autocommit=1
            4 Prepare   select * from test where id= ?
            4 Execute   select * from test where id= 0
            4 Execute   select * from test where id= 1
            4 Execute   select * from test where id= 2
            4 Quit  

另外对这四种情况分别进行5000次查询,执行时间对比

selectWithClientPs span time=232601ms
selectWithClientPsAndCache span time=231493ms
selectWithServerPs span time=233999ms
selectWithServerPsAndCache span time=231262ms

结论:

  1. 使用客户端PreparedStatement,无论是否开启缓存,服务器端都不使用prepare,即硬解析时间不会减少,而且客户端缓存执行时间差别并不大。
  2. 使用服务器端PreparedStatement,如果开启缓存,则会使用prepare,硬解析仅为一次;如果不开启缓存,每次PreparedStatement进行close后,都需要重新进行prepare。
  3. 从测试效果来看,如果SQL本身比较简单,服务器端prepare并没有太大优势,使用客户端prepare即可,如果SQL较为复杂,则可尝试开启服务器端prepare,网上也有文章做过测试称可提高7%性能,不过笔者本地测试提升不到1%,测试数据实际跟SQL的复杂程度有关。

另外还有两个参数

  • prepStmtCacheSize参数,控制缓存的条数,MySQL驱动默认是25,实际使用时一般会根据需要调整大些;
  • prepStmtCacheSqlLimit参数,控制长度多大的SQL可以被缓存,MySQL驱动默认是256,实际使用时如果SQL较大,可调整大些。

关于PrepareStament,MySQL还有一个参数max_prepared_stmt_count,默认值为16382。

mysql> show variables like "max_prepare%"; 
+----------------------------+----------------------+ 
| Variable_name              | Value                | 
+----------------------------+----------------------+ 
| max_prepared_stmt_count    | 16382                | 

如果创建的PS数量超过这个数值,则会报以下错误:

ERROR 1461 (42000): Can’t create more than max_prepared_stmt_count statements

可根据需要需要调大该值,不过调整前应先检查应用是否正确关闭PreparedStament对象。

由上可见,出于历史版本的迭代,MySQL在实现JDBC规范接口时,有很多功能最开始并不算“真正”的实现,而是客户端类facade的设计,因此很多默认功能与常识并不一致,这些需要我们在实际使用中特别注意。

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