17.MyBatis入门系列(17) -MyBatis四大组件之StatementHandler源码及流程解析
17.MyBatis入门系列(17) -MyBatis四大组件之StatementHandler源码及流程解析
前言
上篇文档我们分析了Executor源码及流程解析，我们了解到，Executor是SQL方法的执行器，是SQL方法执行的入口，配合其他组件，完成整个SQL的生命周期。Executor最后执行是由StatementHandler调用Statement去处理的，那么现在，就需要了解下MyBatis另外一个重要组件StatementHandler。
StatementHandler字面上来看，是Statement的处理器。那么什么是Statement呢？
Statement
MyBatis中用的Statement是引入的java.sql.Statement，我们先看下使用原生JDBC操作数据库的部分代码。
Connection conn = null;
Statement stmt = null;
ResultSet rs = null;
try
{
Class.forName(driver);
conn = DriverManager.getConnection(url,user,password);
String sql = "SELECT sNo,sName,sex,age FROM "
+ "Student WHERE dept = '计算机'";
stmt = conn.createStatement();
rs = stmt.executeQuery(sql);
while(rs.next())
{
String no = rs.getString("sNo");
String name = rs.getString("sName");
String sex = rs.getString("sex");
int age = rs.getInt("age");
System.out.println(no + " " + name + " " + sex + " " + age);
}
}从以上代码可以看出，Statement是用来查询SQL及返回结果处理的。
Statement官方定义：用于执行静态 SQL 语句并返回它产生的结果的对象。
Statement接口定义了很多增删改查的方法。
Statement是通过Connection连接对象创建的，Connection也是接口，提供了很多创建Statement及提交回滚的方法，可以看出，Connection可以创建三种Statement对象。
Statement：用于发送简单的SQL语句（不带参数的），如果要使用变量以及其他参数的时候，需要使用+号进行拼接，Statement接口是不安全的，很容易注入数据，将数据丢失或者被人恶意修改！
PreparedStatement：表示预编译 SQL 语句的对象，继承自statement接口，由preparedStatement创建，用于发送一个或多个输入参数的sql语句。prepareedStatement对象的效率更高，并且可以防止SQL注入。一般推荐使用preparedStatement。
CallableStatement：支持调用存储过程,提供了对输出和输入/输出参数(INOUT)的支持。
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。核心是通过#{ } 实现的。
StatementHandler
StatementHandler接口
StatementHandler是一个接口，继承结构如下：
StatementHandler也定义了使用Statement 进行CRUD的相关方法。
/**
* @author Clinton Begin
* Statement 处理器
*/
public interface StatementHandler {
// 用于创建一个具体的 Statement 对象的实现类或者是 Statement 对象
Statement prepare(Connection connection, Integer transactionTimeout)
throws SQLException;
// 用于初始化 Statement 对象以及对sql的占位符进行赋值
void parameterize(Statement statement)
throws SQLException;
// 用于通知 Statement 对象将进行批量操作
void batch(Statement statement)
throws SQLException;
// 用于通知 Statement 对象将 insert、update、delete 操作推送到数据库
int update(Statement statement)
throws SQLException;
// 用于通知 Statement 对象将 select 操作推送数据库并返回对应的查询结果
<E> List<E> query(Statement statement, ResultHandler resultHandler)
throws SQLException;
// 游标查询
<E> Cursor<E> queryCursor(Statement statement)
throws SQLException;
// 获取BoundSql对象
BoundSql getBoundSql();
// 获取ParameterHandler
ParameterHandler getParameterHandler();
}RoutingStatementHandler
RoutingStatementHandler，路由Statement处理器，在XML编写SQL的时候，我们可以在标签中定义statementType，可以取值为STATEMENT，PREPARED，CALLABLE。这几个就对应了JDK中的三种常用Statement。
<select id="selectDynamicUserList" statementType="PREPARED">RoutingStatementHandler的作用，就是根据statementType，生成不同的代理对象，可以在源码中看出：
// 根据MappedStatement配置的StatementType，生成不同的StatementHandler
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
switch (ms.getStatementType()) {
case STATEMENT:
delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
case PREPARED:
delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
case CALLABLE:
delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
default:
throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
}
}BaseStatementHandler
BaseStatementHandler是一个抽象类，实现了StatementHandler接口，用于简化StatementHandler 接口实现的难度,属于适配器设计模式体现。本身并没有实现和CURD相关操作，更多的是设置了一些参数相关。它主要有三个实现类：
SimpleStatementHandler: 管理 Statement 对象并向数据库中推送不需要预编译的SQL语句
PreparedStatementHandler: 管理 Statement 对象并向数据中推送需要预编译的SQL语句，
CallableStatementHandler：管理 Statement 对象并调用数据库中的存储过程
public abstract class BaseStatementHandler implements StatementHandler {
// 全局配置
protected final Configuration configuration;
// 对象工厂
protected final ObjectFactory objectFactory;
// 类型处理器
protected final TypeHandlerRegistry typeHandlerRegistry;
// 结果集处理器
protected final ResultSetHandler resultSetHandler;
// 参数处理器
protected final ParameterHandler parameterHandler;
// 执行器
protected final Executor executor;
// mapper的SQL对象
protected final MappedStatement mappedStatement;
// 分页参数
protected final RowBounds rowBounds;
// 绑定SQL对象
protected BoundSql boundSql;
// 构造方法
protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
this.configuration = mappedStatement.getConfiguration();
this.executor = executor;
this.mappedStatement = mappedStatement;
this.rowBounds = rowBounds;
this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
this.objectFactory = configuration.getObjectFactory();
if (boundSql == null) { // issue #435, get the key before calculating the statement
generateKeys(parameterObject);
boundSql = mappedStatement.getBoundSql(parameterObject);
}
this.boundSql = boundSql;
this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
}
@Override
public BoundSql getBoundSql() {
return boundSql;
}
@Override
public ParameterHandler getParameterHandler() {
return parameterHandler;
}
// 预编译SQL语句
@Override
public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
ErrorContext.instance().sql(boundSql.getSql());
Statement statement = null;
try {
statement = instantiateStatement(connection);
// //设置statement超时时间
setStatementTimeout(statement, transactionTimeout);
// //设置statement的fetchSize
setFetchSize(statement);
return statement;
} catch (SQLException e) {
closeStatement(statement);
throw e;
} catch (Exception e) {
closeStatement(statement);
throw new ExecutorException("Error preparing statement. Cause: " + e, e);
}
}
// 调用抽象方法构建statement对象是，但是没有具体实现，而是交给其子类去实现
protected abstract Statement instantiateStatement(Connection connection) throws SQLException;
// 给statement对象设置timeout
protected void setStatementTimeout(Statement stmt, Integer transactionTimeout) throws SQLException {
Integer queryTimeout = null;
if (mappedStatement.getTimeout() != null) {
queryTimeout = mappedStatement.getTimeout();
} else if (configuration.getDefaultStatementTimeout() != null) {
queryTimeout = configuration.getDefaultStatementTimeout();
}
if (queryTimeout != null) {
stmt.setQueryTimeout(queryTimeout);
}
StatementUtil.applyTransactionTimeout(stmt, queryTimeout, transactionTimeout);
}
// 给statement对象设置fetchSize
protected void setFetchSize(Statement stmt) throws SQLException {
Integer fetchSize = mappedStatement.getFetchSize();
if (fetchSize != null) {
stmt.setFetchSize(fetchSize);
return;
}
Integer defaultFetchSize = configuration.getDefaultFetchSize();
if (defaultFetchSize != null) {
stmt.setFetchSize(defaultFetchSize);
}
}
// 关闭Statement
protected void closeStatement(Statement statement) {
try {
if (statement != null) {
statement.close();
}
} catch (SQLException e) {
//ignore
}
}
// 主键生成
protected void generateKeys(Object parameter) {
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
ErrorContext.instance().store();
keyGenerator.processBefore(executor, mappedStatement, null, parameter);
ErrorContext.instance().recall();
}
}SimpleStatementHandler
SimpleStatementHandler就是使用基本的Statement来执行query、batch、update等操作。
/**
* @author Clinton Begin
* 简单的StatementHandler
*/
public class SimpleStatementHandler extends BaseStatementHandler {
public SimpleStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
}
// 更新
@Override
public int update(Statement statement) throws SQLException {
// 获得sql语句
String sql = boundSql.getSql();
// 获得参数
Object parameterObject = boundSql.getParameterObject();
// 获取主键生成
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
int rows;
// statement执行sql语句返回更新数目
if (keyGenerator instanceof Jdbc3KeyGenerator) {
statement.execute(sql, Statement.RETURN_GENERATED_KEYS);
rows = statement.getUpdateCount();
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else if (keyGenerator instanceof SelectKeyGenerator) {
statement.execute(sql);
rows = statement.getUpdateCount();
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else {
// 如果没有keyGenerator，直接调用Statement.execute和Statement.getUpdateCount
statement.execute(sql);
rows = statement.getUpdateCount();
}
return rows;
}
// 批处理
@Override
public void batch(Statement statement) throws SQLException {
String sql = boundSql.getSql();
statement.addBatch(sql);
}
// statement查询
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
String sql = boundSql.getSql();
statement.execute(sql);
return resultSetHandler.<E>handleResultSets(statement);
}
@Override
public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
String sql = boundSql.getSql();
statement.execute(sql);
return resultSetHandler.<E>handleCursorResultSets(statement);
}
@Override
protected Statement instantiateStatement(Connection connection) throws SQLException {
if (mappedStatement.getResultSetType() != null) {
return connection.createStatement(mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
} else {
return connection.createStatement();
}
}
// 由于SimpleStatementHandler是处理没有参数的SQL，所以参数设置的方法无需任何处理
@Override
public void parameterize(Statement statement) throws SQLException {
// N/A
}
}PreparedStatementHandler
PreparedStatementHandler是我们最常用的Statement处理器，主要是创建PrepareStatement来执行CRUD，虽然初次创建PrepareStatement时开销比较大，但在多次处理SQL时只需要初始化一次，可以有效提高性能。
源码和SimpleStatementHandler差不多，只是使用的Statement是PrepareStatement。
/**
* @author Clinton Begin
*/
/* 使用PrepareStatement**/
public class PreparedStatementHandler extends BaseStatementHandler {
public PreparedStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
}
//
public int update(Statement statement) throws SQLException {
//使用PrepareStatement
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
int rows = ps.getUpdateCount();
Object parameterObject = boundSql.getParameterObject();
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
//keyGenerator在执行之后运行
keyGenerator.processAfter(executor, mappedStatement, ps, parameterObject);
return rows;
}
//使用PrepareStatement的批处理
public void batch(Statement statement) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.addBatch();
}
//使用PrepareStatement的execute操作
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
//操作结果在ResultHandler中处理
return resultSetHandler.<E> handleResultSets(ps);
}
//获得Statement
protected Statement instantiateStatement(Connection connection) throws SQLException {
String sql = boundSql.getSql();
//根据KeyGenerator设置值的返回key的名称
if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
String[] keyColumnNames = mappedStatement.getKeyColumns();
if (keyColumnNames == null) {
return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
} else {
return connection.prepareStatement(sql, keyColumnNames);
}
} else if (mappedStatement.getResultSetType() != null) {
return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
} else {
return connection.prepareStatement(sql);
}
}
public void parameterize(Statement statement) throws SQLException {
parameterHandler.setParameters((PreparedStatement) statement);
}
}CallableStatementHandler
CallableStatementHandler实际就是使用CallableStatement来执行SQL语句，当然它执行的是存储过程。一般也很少使用。
public class CallableStatementHandler extends BaseStatementHandler {
public CallableStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
}
@Override
public int update(Statement statement) throws SQLException {
//用来调用存储过程,它提供了对输出和输入/输出参数的支持
CallableStatement cs = (CallableStatement) statement;
cs.execute();
int rows = cs.getUpdateCount();
Object parameterObject = boundSql.getParameterObject();
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
keyGenerator.processAfter(executor, mappedStatement, cs, parameterObject);
resultSetHandler.handleOutputParameters(cs);
return rows;
}
@Override
public void batch(Statement statement) throws SQLException {
CallableStatement cs = (CallableStatement) statement;
cs.addBatch();
}
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
CallableStatement cs = (CallableStatement) statement;
cs.execute();
List<E> resultList = resultSetHandler.<E>handleResultSets(cs);
resultSetHandler.handleOutputParameters(cs);
return resultList;
}
@Override
public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
CallableStatement cs = (CallableStatement) statement;
cs.execute();
Cursor<E> resultList = resultSetHandler.<E>handleCursorResultSets(cs);
resultSetHandler.handleOutputParameters(cs);
return resultList;
}
@Override
protected Statement instantiateStatement(Connection connection) throws SQLException {
String sql = boundSql.getSql();
if (mappedStatement.getResultSetType() != null) {
return connection.prepareCall(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
} else {
return connection.prepareCall(sql);
}
}
@Override
public void parameterize(Statement statement) throws SQLException {
//注册out参数
registerOutputParameters((CallableStatement) statement);
parameterHandler.setParameters((CallableStatement) statement);
}
private void registerOutputParameters(CallableStatement cs) throws SQLException {
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
for (int i = 0, n = parameterMappings.size(); i < n; i++) {
ParameterMapping parameterMapping = parameterMappings.get(i);
//处理存储过程的INOUT和OUT
if (parameterMapping.getMode() == ParameterMode.OUT || parameterMapping.getMode() == ParameterMode.INOUT) {
if (null == parameterMapping.getJdbcType()) {
throw new ExecutorException("The JDBC Type must be specified for output parameter. Parameter: " + parameterMapping.getProperty());
} else {
if (parameterMapping.getNumericScale() != null && (parameterMapping.getJdbcType() == JdbcType.NUMERIC || parameterMapping.getJdbcType() == JdbcType.DECIMAL)) {
cs.registerOutParameter(i + 1, parameterMapping.getJdbcType().TYPE_CODE, parameterMapping.getNumericScale());
} else {
if (parameterMapping.getJdbcTypeName() == null) {
cs.registerOutParameter(i + 1, parameterMapping.getJdbcType().TYPE_CODE);
} else {
cs.registerOutParameter(i + 1, parameterMapping.getJdbcType().TYPE_CODE, parameterMapping.getJdbcTypeName());
}
}
}
}
}
}
}StatementHandler流程分析
执行流程
1. 创建StatementHandler
StatementHandler是在Executor执行器进行CRUD的时候进行创建的。比如在SimpleExecutor进行查询doQuery的时候：
// select
@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
// Statement
Statement stmt = null;
try {
// 获取配置
Configuration configuration = ms.getConfiguration();
// 创建StatementHandler
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
// 准备Statement
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.<E>query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}执行器会调用配置对象的newStatementHandler方法进行创建StatementHandler。
/**
* 创建 StatementHandler
*
* @param executor 执行器
* @param mappedStatement MS
* @param parameterObject 参数
* @param rowBounds 分页
* @param resultHandler 结果处理器
* @param boundSql 绑定SQL 对象
* @return
*/
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
// 创建RoutingStatementHandler
StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
// 所有拦截器进行包装
statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
return statementHandler;
}创建StatementHandler时，创建的都是RoutingStatementHandler，这是一个包装类，会根据不同的配置，生成不同的StatementHandler实例对象。
// 根据MappedStatement配置的StatementType，生成不同的StatementHandler
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
switch (ms.getStatementType()) {
// STATEMENT
case STATEMENT:
delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
case PREPARED:
delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
case CALLABLE:
delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
default:
throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
}
}我们这里使用的是PREPARED，所以会进入new PreparedStatementHandler方法内，PreparedStatementHandler的构造方法又是调用父类的构造，调用的是BaseStatementHandler的构造方法。
构造方法会对当前PreparedStatementHandler设置相关参数，比如configuration、executor、mappedStatement、typeHandlerRegistry等，并会创建重要的另外两个处理器（ParameterHandler、ResultSetHandler），后续介绍。
RoutingStatementHandler创建之后，也会使用所有的拦截器对其进行包装。
2. 准备语句
执行器中的StatementHandler创建之后，就会调用prepareStatement方法，就当前StatementHandler进行解析，准备语句。
stmt = this.prepareStatement(handler, ms.getStatementLog()); // 准备语句
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
// 获取连接
Connection connection = getConnection(statementLog);
// 调用StatementHandler.prepare
stmt = handler.prepare(connection, transaction.getTimeout());
// 调用StatementHandler.parameterize
handler.parameterize(stmt);
return stmt;
}首先是获取数据库连接，我这里获取到的Hikari连接池中的连接，获取到连接之后，调用handler的prepare方法，实例化statement，及设置相关参数。
// 预编译SQL语句
@Override
public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
ErrorContext.instance().sql(boundSql.getSql());
Statement statement = null;
try {
statement = instantiateStatement(connection);
// //设置statement超时时间
setStatementTimeout(statement, transactionTimeout);
// //设置statement的fetchSize
setFetchSize(statement);
return statement;
} catch (SQLException e) {
closeStatement(statement);
throw e;
} catch (Exception e) {
closeStatement(statement);
throw new ExecutorException("Error preparing statement. Cause: " + e, e);
}
}instantiateStatement方法调用JDK中的Connection接口进行创建Statement。
// 构造Statement对象
@Override
protected Statement instantiateStatement(Connection connection) throws SQLException {
// 通过Connection来create一个Statement对象
if (mappedStatement.getResultSetType() != null) {
return connection.createStatement(mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
} else {
return connection.createStatement();
}
}3. JDBC查询
Statement创建成功后，接着调用参数处理器对参数进行处理，Statement彻底完善细节后返回。之后就开始使用处理器执行查询操作了，实际使用的还是JDBC包中的执行方法。
var9 = handler.query(stmt, resultHandler);
4. 结果集处理返回
JDBC操作完成后，返回PreparedStatement，并调用结果处理器进行结果集处理。
结果处理器处理完后，返回查询结果，关闭statement，整个流程也就结束了。
来源：https://yunyanchengyu.blog.csdn.net/article/details/118227115