前言
大家都知道,Mybatis内置参数,形如#{xxx}
的,均采用了sql预编译的形式,大致知道Mybatis底层使用PreparedStatement
,过程是先将带有占位符(即”?”)的sql模板发送至MySQL服务器,由服务器对此无参数的sql进行编译后,将编译结果缓存,然后直接执行带有真实参数的sql。如果基本结论也是如此,那就大错特错了。
目录
- db是否支持预编译
- 连接数据库的url是否指定了需要预编译,比如:
jdbc:mysql://127.0.0.1:3306/user?useServerPrepStmts=true
,useServerPrepStmts=true
是非常非常重要的参数。如果不配置PreparedStatement
实际是个假的PreparedStatement
```sql SELECT VERSION(); // 5.6.24-log
SHOW GLOBAL STATUS LIKE ‘%prepare%’; //Com_stmt_prepare 4 代表被执行预编译次数
//开启server日志 SHOW VARIABLES LIKE ‘%general_log%’; SHOW VARIABLES LIKE ‘log_output’;
SET GLOBAL general_log = ON; SET GLOBAL log_output=’table’;
TRUNCATE TABLE mysql.general_log;
SELECT * FROM mysql.general_log; // 有Prepare命令
注意:MySQL预编译功能有版本要求,包括server版本和mysql.jar包版本。以前的版本默认`useServerPrepStmts=true`,5.0.5以后的版本默认`useServerPrepStmts=false`
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## 2、预编译缓存是服务端还是客户端缓存?
开启缓存:`useServerPrepStmts=true&cachePrepStmts=true`,设置了`useServerPrepStmts=true`,虽然可以一次编译,多次执行它可以提高性能,但缓存是针对连接的,即每个连接的缓存都是独立的,并且缓存主要是由mysql-connector-java.jar实现的。<br />当手动调用`prepareStatement.close()`时`PrepareStatement`对象只会将关闭状态置为关闭,并不会向MySQL发送关闭请求,`prepareStatement`对象会被缓存起来,等下次使用的时候直接从缓存中取出来使用。没有开启缓存,则会向MySQL发送`closeStmt`的请求。
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## 3、开启预编译性能更高?
也就是说预编译比非预编译更好?其实不然,不行自己可试试看。
```java
public class PreparedStatement_test {
private String url = "jdbc:mysql://localhost:3306/batch";
private String sql = "SELECT * FROM export_request WHERE id = ?";
private int maxTimes = 100000;
@Test
public void go_driver() throws SQLException, ClassNotFoundException {
Class.forName("com.mysql.jdbc.Driver");
Connection conn = (Connection) DriverManager.getConnection(url, "root", "123456");
// PreparedStatement
Stopwatch stopwatch = Stopwatch.createStarted();
for (int i = 0; i < maxTimes; i++) {
PreparedStatement stmt = conn.prepareStatement(sql);
stmt.setLong(1, Math.abs(new Random().nextLong()));
// execute
stmt.executeQuery();
}
System.out.println("go_driver:" + stopwatch);
}
@Test
public void go_setPre() throws SQLException, ClassNotFoundException {
Class.forName("com.mysql.jdbc.Driver");
Connection conn = (Connection) DriverManager.getConnection(url + "?useServerPrepStmts=true", "root", "123456");
// PreparedStatement
Stopwatch stopwatch = Stopwatch.createStarted();
for (int i = 0; i < maxTimes; i++) {
PreparedStatement stmt = conn.prepareStatement(sql);
stmt.setLong(1, Math.abs(new Random().nextLong()));
// execute
stmt.executeQuery();
}
System.out.println("go_setPre:" + stopwatch);
}
@Test
public void go_setPreCache() throws SQLException, ClassNotFoundException {
Class.forName("com.mysql.jdbc.Driver");
Connection conn = (Connection) DriverManager.getConnection(url + "?useServerPrepStmts=true&cachePrepStmts=true", "root", "123456");
// PreparedStatement
PreparedStatement stmt = conn.prepareStatement(sql);
stmt.setLong(1, Math.abs(new Random().nextLong()));
// execute
stmt.executeQuery();
stmt.close();//非常重要的,一定要调用才会缓存
Stopwatch stopwatch = Stopwatch.createStarted();
for (int i = 0; i < maxTimes; i++) {
stmt = conn.prepareStatement(sql);
stmt.setLong(1, Math.abs(new Random().nextLong()));
// execute
stmt.executeQuery();
}
System.out.println("go_setPreCache:" + stopwatch);
}
}
基准为10w次单线程:
- 非预编译::23.78 s
- 预编译:41.86 s
- 预编译缓存:20.55 s
经过实践测试,对于频繁适用的语句,使用预编译+缓存确实能够得到可观的提升,但对于不频繁适用的语句,服务端编译会增加额外的round-trip。开发实践中要视情况而定。
4、从源码中验证
预编译原理(connection -> prepareStatement )
预编译:JDBC42ServerPreparedStatement
(需将对应占位符)
非预编译:JDBC42PreparedStatement
(完整的SQL)
//com.mysql.jdbc.ConnectionImpl中的代码片段
/**
* JDBC 2.0 Same as prepareStatement() above, but allows the default result
* set type and result set concurrency type to be overridden.
*
* @param sql
* the SQL query containing place holders
* @param resultSetType
* a result set type, see ResultSet.TYPE_XXX
* @param resultSetConcurrency
* a concurrency type, see ResultSet.CONCUR_XXX
* @return a new PreparedStatement object containing the pre-compiled SQL
* statement
* @exception SQLException
* if a database-access error occurs.
*/
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
//当Client开启 useServerPreparedStmts 并且Server支持 ServerPrepare
PreparedStatement pStmt = null;
boolean canServerPrepare = true;
String nativeSql = getProcessEscapeCodesForPrepStmts() ? nativeSQL(sql) : sql;
if (this.useServerPreparedStmts && getEmulateUnsupportedPstmts()) {
canServerPrepare = canHandleAsServerPreparedStatement(nativeSql);
}
if (this.useServerPreparedStmts && canServerPrepare) {// 从缓存中获取 pStmt
if (this.getCachePreparedStatements()) {
synchronized (this.serverSideStatementCache) {
pStmt = (com.mysql.jdbc.ServerPreparedStatement) this.serverSideStatementCache
.remove(makePreparedStatementCacheKey(this.database, sql));
if (pStmt != null) {
((com.mysql.jdbc.ServerPreparedStatement) pStmt).setClosed(false);
pStmt.clearParameters();// 清理上次留下的参数
}
if (pStmt == null) {
try {// 向Server提交 SQL 预编译,实例是JDBC42ServerPreparedStatement
pStmt = ServerPreparedStatement.getInstance(getMultiHostSafeProxy(), nativeSql, this.database, resultSetType,
resultSetConcurrency);
if (sql.length() < getPreparedStatementCacheSqlLimit()) {
((com.mysql.jdbc.ServerPreparedStatement) pStmt).isCached = true;
}
pStmt.setResultSetType(resultSetType);
pStmt.setResultSetConcurrency(resultSetConcurrency);
} catch (SQLException sqlEx) {
// Punt, if necessary
if (getEmulateUnsupportedPstmts()) {
pStmt = (PreparedStatement) clientPrepareStatement(nativeSql, resultSetType, resultSetConcurrency, false);
if (sql.length() < getPreparedStatementCacheSqlLimit()) {
this.serverSideStatementCheckCache.put(sql, Boolean.FALSE);
}
} else {
throw sqlEx;
}
}
}
}
} else {
try { // 向Server提交 SQL 预编译。
pStmt = ServerPreparedStatement.getInstance(getMultiHostSafeProxy(), nativeSql, this.database, resultSetType, resultSetConcurrency);
pStmt.setResultSetType(resultSetType);
pStmt.setResultSetConcurrency(resultSetConcurrency);
} catch (SQLException sqlEx) {
// Punt, if necessary
if (getEmulateUnsupportedPstmts()) {
pStmt = (PreparedStatement) clientPrepareStatement(nativeSql, resultSetType, resultSetConcurrency, false);
} else {
throw sqlEx;
}
}
}
} else {// Server不支持 ServerPrepare,实例是JDBC42PreparedStatement
pStmt = (PreparedStatement) clientPrepareStatement(nativeSql, resultSetType, resultSetConcurrency, false);
}
return pStmt;
}
}
JDBC42ServerPreparedStatement->close,缓存
//com.mysql.jdbc.ServerPreparedStatement中选取代码
@Override
public void close() throws SQLException {
MySQLConnection locallyScopedConn = this.connection;
if (locallyScopedConn == null) {
return; // already closed
}
synchronized (locallyScopedConn.getConnectionMutex()) {
if (this.isCached && isPoolable() && !this.isClosed) {
clearParameters();// 若开启缓存,则只会将状态位设为已关闭,并且刷新缓存
this.isClosed = true;
this.connection.recachePreparedStatement(this);
return;
}
//没有开启缓存,则会向mysql发送closeStmt的请求
realClose(true, true);
}
}
public void recachePreparedStatement(ServerPreparedStatement pstmt) throws SQLException {
synchronized (getConnectionMutex()) {
if (getCachePreparedStatements() && pstmt.isPoolable()) {
synchronized (this.serverSideStatementCache) {
Object oldServerPrepStmt = this.serverSideStatementCache.put(makePreparedStatementCacheKey(pstmt.currentCatalog, pstmt.originalSql), pstmt);
if (oldServerPrepStmt != null) {// 将sql语句作为key,reparedStatement对象作为value存放到缓存中
((ServerPreparedStatement) oldServerPrepStmt).isCached = false;
((ServerPreparedStatement) oldServerPrepStmt).realClose(true, true);
}
}
}
}
}
5、总结
- 预编译显式开启(在url中指定
useServerPrepStmts=true
),否则PreparedStatement
不会向MySQL发送预编译(Prepare命令)的请求; - 每次向MySQL发送预编译请求,不管之前有没有执行过此SQL语句,只要请求的命令是Prepare或Query,MySQL就会重新编译一次SQL语句,并返回此链接当前唯一的Statement ID,后续执行SQL语句的时候,程序只需拿着Statement ID和参数就可以了;
- 当预编译的SQL语句有语法错误,则MySQL的响应会携带错误信息,但此错误信息JDBC感知不到(或者说mysql-connetor-java.jar包里的实现将其忽略掉了),此时还会继续往下执行代码,当执行到
executeXxx()
方法时,由于没有Statement ID(所以就会将拼接完整的SQL语句值已经将占位符(?)替换掉再次发给MySQL请求执行,此时MySQL响应有语法错误,这时JDBC就会抛出语法错误异常),所以检查语法那一步实在mysql-server中做的(通过抓包可以看到); PreparedStatement
对性能的提高是利用缓存实现的,需要显式开启(在url中指定cachePrepStmts=true
),此缓存是mysql-connetor-java.jar包里实现的(非mysql-server中的缓存),缓存的key是完整的sql语句,value是PreparedStatement对象。放入缓存是PreparedStatement.close()
触发的,所以只要缓存PreparedStatement
对象没有关闭,不管调用多少次connection.prapareStatement(sql)
对相同的sql语句进行预编译,都会将预编译的请求发给MySQL,MySQL也会对每一个sql语句不管是否相同进行预编译,并生成一个唯一的Statement ID并返回;- 缓存是针对链接的,每个链接都是独立的,不共享缓存。