java培训 | Mybatis 中的 PreparedStatement 预编译

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。如果你的基本结论也是如此，那你就大错特错了。

1. mysql是否默认开启了预编译功能？

mysql是否支持预编译有两层意思：

• 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

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

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。

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，【关注尚硅谷，轻松学IT】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。

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。

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，www.atguigu.com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，你不管调用多少次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

文章来源于Java知音