Mybatis工作流程及其原理与解析 / 四六文摘

Mybatis简介：

MyBatis 是一款优秀的持久层框架，它支持定制化 SQL、存储过程以及高级映射。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以使用简单的 XML 或注解来配置和映射原生信息，将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。本文将通过debug的方式来了解其工作原理。

Mybatis核心类：

SqlSessionFactory：每个基于 MyBatis 的应用都是以一个 SqlSessionFactory 的实例为中心的。SqlSessionFactory 的实例可以通过 SqlSessionFactoryBuilder 获得。而 SqlSessionFactoryBuilder 则可以从 XML 配置文件或通过Java的方式构建出 SqlSessionFactory 的实例。SqlSessionFactory 一旦被创建就应该在应用的运行期间一直存在，建议使用单例模式或者静态单例模式。一个SqlSessionFactory对应配置文件中的一个环境（environment），如果你要使用多个数据库就配置多个环境分别对应一个SqlSessionFactory。

SqlSession：SqlSession是一个接口，它有2个实现类，分别是DefaultSqlSession(默认使用)以及SqlSessionManager。SqlSession通过内部存放的执行器（Executor）来对数据进行CRUD。此外SqlSession不是线程安全的，因为每一次操作完数据库后都要调用close对其进行关闭，官方建议通过try-finally来保证总是关闭SqlSession。

Executor：Executor（执行器）接口有两个实现类，其中BaseExecutor有三个继承类分别是BatchExecutor（重用语句并执行批量更新），ReuseExecutor（重用预处理语句prepared statement，跟Simple的唯一区别就是内部缓存statement），SimpleExecutor（默认，每次都会创建新的statement）。以上三个就是主要的Executor。通过下图可以看到Mybatis在Executor的设计上面使用了装饰器模式，我们可以用CachingExecutor来装饰前面的三个执行器目的就是用来实现缓存。

MappedStatement：MappedStatement就是用来存放我们SQL映射文件中的信息包括sql语句，输入参数，输出参数等等。一个SQL节点对应一个MappedStatement对象。

Mybatis工作流程：

下面将通过debug方式对Mybatis进行一步步解析。首先贴出我的mybatis-config.xml文件以及Mapper.xml文件。

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<properties resource="jdbc.properties"></properties>
<settings>
<setting name="cacheEnabled" value="true"/>
<setting name="defaultExecutorType" value="REUSE"/>
</settings>
<typeAliases>
<typeAlias alias="User" type="com.ctc.model.User"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<package name="com.ctc.mapper"/>
</mappers>
</configuration>

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.ctc.mapper.UserMapper">
<cache readOnly="true" size="200" eviction="FIFO"></cache>
<sql id="select"> select * from user </sql>
<select id="selectUser" resultMap="selectUserMap" useCache="true">
<include refid="select"></include> User where id = #{id}
</select>
<resultMap type="User" id="selectUserMap">
<result property="name" column="username"/>
</resultMap>
<insert id="insertUser" useGeneratedKeys="true" keyProperty="id" keyColumn="id">
insert into User (username,birthday,sex,address)
values (#{name},#{birthday},#{sex},#{address})
</insert>
<update id="updateUser">
update User set username = #{username},birthday = #{birthday},
sex = #{sex},address = #{address} where id = #{id}
</update>
<delete id="deleteUser" >
delete from User where id = #{id}
</delete>
<select id="selectUserByName" resultMap="selectUserMap">
select * from User where sex = #{param1}
<choose>
<when test="{param2} != null">
and username like #{param2}
</when>
<otherwise>and address = #{parma3}</otherwise>
</choose>
</select>
<select id="selectUserCount" resultType="int" >
select count(*) from user where username like #{username}
</select>
<select id="selectUserNew" resultMap="selectUserMap">
<bind name="pattern" value="'%' + name + '%'" />
<include refid="select"></include>
<where>
<if test="pattern !=null">
username like #{pattern}
</if>
<if test="sex !=null">
and sex = #{sex}
</if>
<if test="address != null">
and address = #{address}
</if>
</where>
</select>
<select id="selectUserByIds" resultMap="selectUserMap">
<include refid="select"></include>
where id in
<foreach collection="list" item="id" index="0" open="(" close=")" separator="," >
#{id}
</foreach>
</select>
</mapper>

第一步通过SqlSessionFactoryBuilder创建SqlSessionFactory：

首先在SqlSessionFactoryBuilder的build（）方法中可以看到MyBatis内部定义了一个类XMLConfigBuilder用来解析配置文件mybatis-config.xml。针对配置文件中的每一个节点进行解析并将数据存放到Configuration这个对象中，紧接着使用带有Configuration的构造方法发返回一个DefautSqlSessionFactory。

public SqlSessionFactory build(InputStream inputStream) {
return build(inputStream, null, null);
}
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
//解析mybatis-config.xml
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
inputStream.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}
//返回SqlSessionFactory，默认使用的是实现类DefaultSqlSessionFactory
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
//获取根节点configuration
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
//开始解析mybatis-config.xml,并把解析后的数据存放到configuration中
private void parseConfiguration(XNode root) {
try {
//保存mybatis-config.xml中的标签setting,本例中开启全局缓存cacheEnabled，设置默认执行器defaultExecutorType=REUSE
Properties settings = settingsAsPropertiess(root.evalNode("settings"));
//issue #117 read properties first
//解析是否配置了外部properties，例如本例中配置的jdbc.propertis
propertiesElement(root.evalNode("properties"));
//查看是否配置了VFS，默认没有，本例也没有使用
loadCustomVfs(settings);
//查看是否用了类型别名，减少完全限定名的冗余，本例中使用了别名User代替了com.ctc.Model.User
typeAliasesElement(root.evalNode("typeAliases"));
//查看是否配置插件来拦截映射语句的执行，例如拦截Executor的Update方法，本例没有使用
pluginElement(root.evalNode("plugins"))
//查看是否配置了ObjectFactory，默认情况下使用对象的无参构造方法或者是带有参数的构造方法，本例没有使用
objectFactoryElement(root.evalNode("objectFactory"));
//查看是否配置了objectWrapperFatory,这个用来或者ObjectWapper，可以访问：对象，Collection，Map属性。本例没有使用
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
//查看是否配置了reflectorFactory,mybatis的反射工具，提供了很多反射方法。本例没有使用
reflectorFactoryElement(root.evalNode("reflectorFactory"));
//放入参数到configuration对象中
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
//查看数据库环境配置
environmentsElement(root.evalNode("environments"));
//查看是否使用多种数据库，本例没有使用
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
//查看是否配置了新的类型处理器，如果跟处理的类型跟默认的一致就会覆盖。本例没有使用
typeHandlerElement(root.evalNode("typeHandlers"));
//查看是否配置SQL映射文件,有四种配置方式，resource，url，class以及自动扫包package。本例使用package
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}

第二步通过SqlSessionFactory创建SqlSession：

@Override
public SqlSession openSession() {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
//拿到前文从mybatis中解析到的数据库环境配置
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
//拿到jdbc的事务管理器，有两种一种是jbc,一种的managed。本例使用的是JdbcTransaction
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
//从mybatis配置文件可以看到本例使用了REUSE，因此返回的是ReuseExecutor并把事务传入对象中
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
if (ExecutorType.BATCH == executorType) {
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
executor = new SimpleExecutor(this, transaction);
}
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
//返回一个SqlSession，默认使用DefaultSqlSession
public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
this.configuration = configuration;
this.executor = executor;
this.dirty = false;
this.autoCommit = autoCommit;
}

第三步通过SqlSession拿到Mapper对象的代理：

@Override
public <T> T getMapper(Class<T> type) {
return configuration.<T>getMapper(type, this);
}
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//前文解析Mybatis-config.xml的时候，在解析标签mapper就是用configuration对象的mapperRegistry存放数据
return mapperRegistry.getMapper(type, sqlSession);
}
@SuppressWarnings("unchecked")
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//knownMapper是一个HashMap在存放mapperRegistry的过程中，以每个Mapper对象的类型为Key, MapperProxyFactory 为value保存。
//例如本例中保存的就是Key:com.ctc.mapper.UserMapper,value就是保存了key的MapperProxyFactory对象
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
public T newInstance(SqlSession sqlSession) {
//生成一个mapperProxy对象，这个对象实现了InvocationHandler, Serializable。就是JDK动态代理中的方法调用处理器
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
//通过JDK动态代理生成一个Mapper的代理，在本例中的就是UserMapper的代理类，它实现了UserMapper接口
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}

第四步通过MapperProxy调用Maper中相应的方法：

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//判断当前调用的method是不是Object中声明的方法，如果是的话直接执行。
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
//把当前请求放入一个HashMap中，一旦下次还是同样的方法进来直接返回。
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
switch (command.getType()) {
case INSERT: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
case DELETE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
case SELECT:
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {
result = executeForCursor(sqlSession, args);
} else {
//本次案例会执行selectOne
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}
@Override
public <T> T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
List<T> list = this.<T>selectList(statement, parameter);
if (list.size() == 1) {
return list.get(0);
} else if (list.size() > 1) {
throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
} else {
return null;
}
}
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
//这边调用的是CachingExecutor类的query,还记得前文解析mybatis-config.xml的时候我们指定了REUSE但是因为在配置文件中开启了缓存
//所以ReuseExecutor被CachingExecotur装饰，新增了缓存的判断，最后还是会调用ReuseExecutor
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject);
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
Cache cache = ms.getCache();
if (cache != null) {
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
ensureNoOutParams(ms, parameterObject, boundSql);
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
//如果缓存中没有数据则查询数据库
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
//结果集放入缓存
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

Mybatis工作流程及其原理与解析

Mybatis简介：

Mybatis核心类：

Mybatis工作流程：

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