MyBatis执行Sql的流程实例解析

这篇文章主要介绍了MyBatis执行Sql的流程实例解析,文中通过示例代码介绍的非常详细，对大家的学习或者工作具有一定的参考学习价值,需要的朋友可以参考下

本博客着重介绍MyBatis执行Sql的流程，关于在执行过程中缓存、动态SQl生成等细节不在本博客中体现，相应内容后面再单独写博客分析吧。

还是以之前的查询作为列子：

public class UserDaoTest {

  private SqlSessionFactory sqlSessionFactory;

  @Before
  public void setUp() throws Exception{
    ClassPathResource resource = new ClassPathResource("mybatis-config.xml");
    InputStream inputStream = resource.getInputStream();
    sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
  }

  @Test
  public void selectUserTest(){
    String id = "{0003CCCA-AEA9-4A1E-A3CC-06D884BA3906}";
    SqlSession sqlSession = sqlSessionFactory.openSession();
    CbondissuerMapper cbondissuerMapper = sqlSession.getMapper(CbondissuerMapper.class);
    Cbondissuer cbondissuer = cbondissuerMapper.selectByPrimaryKey(id);
    System.out.println(cbondissuer);
    sqlSession.close();
  }

}

之前提到拿到sqlSession之后就能进行各种CRUD操作了，所以我们就从sqlSession.getMapper这个方法开始分析，看下整个Sql的执行流程是怎么样的。

获取Mapper

进入sqlSession.getMapper方法，会发现调的是Configration对象的getMapper方法：

public <T> T getMapper(Class<T> type,SqlSession sqlSession) {
  //mapperRegistry实质上是一个Map，里面注册了启动过程中解析的各种Mapper.xml
  //mapperRegistry的key是接口的全限定名，比如com.csx.demo.spring.boot.dao.SysUserMapper
  //mapperRegistry的Value是MapperProxyFactory,用于生成对应的MapperProxy（动态代理类）
  return mapperRegistry.getMapper(type,sqlSession);
}

进入getMapper方法：

public <T> T getMapper(Class<T> type,SqlSession sqlSession) {
  final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
  //如果配置文件中没有配置相关Mapper,直接抛异常
  if (mapperProxyFactory == null) {
   throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
  }
  try {
   //关键方法
   return mapperProxyFactory.newInstance(sqlSession);
  } catch (Exception e) {
   throw new BindingException("Error getting mapper instance. Cause: " + e,e);
  }
 }

进入MapperProxyFactory的newInstance方法：

public class MapperProxyFactory<T> {

 private final Class<T> mapperInterface;
 private final Map<Method,MapperMethod> methodCache = new ConcurrentHashMap<Method,MapperMethod>();

 public MapperProxyFactory(Class<T> mapperInterface) {
  this.mapperInterface = mapperInterface;
 }

 public Class<T> getMapperInterface() {
  return mapperInterface;
 }

 public Map<Method,MapperMethod> getMethodCache() {
  return methodCache;
 }

 //生成Mapper接口的动态代理类MapperProxy
 @SuppressWarnings("unchecked")
 protected T newInstance(MapperProxy<T> mapperProxy) {
  return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(),new Class[] { mapperInterface },mapperProxy);
 }

 public T newInstance(SqlSession sqlSession) {
  final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession,mapperInterface,methodCache);
  return newInstance(mapperProxy);
 }

}

下面是动态代理类MapperProxy，调用Mapper接口的所有方法都会先调用到这个代理类的invoke方法（注意由于Mybatis中的Mapper接口没有实现类，所以MapperProxy这个代理对象中没有委托类，也就是说MapperProxy干了代理类和委托类的事情）。好了下面重点看下invoke方法。

//MapperProxy代理类
public class MapperProxy<T> implements InvocationHandler,Serializable {

 private static final long serialVersionUID = -6424540398559729838L;
 private final SqlSession sqlSession;
 private final Class<T> mapperInterface;
 private final Map<Method,MapperMethod> methodCache;

 public MapperProxy(SqlSession sqlSession,Class<T> mapperInterface,Map<Method,MapperMethod> methodCache) {
  this.sqlSession = sqlSession;
  this.mapperInterface = mapperInterface;
  this.methodCache = methodCache;
 }

 @Override
 public Object invoke(Object proxy,Method method,Object[] args) throws Throwable {
  try {
   if (Object.class.equals(method.getDeclaringClass())) {
    return method.invoke(this,args);
   } else if (isDefaultMethod(method)) {
    return invokeDefaultMethod(proxy,method,args);
   }
  } catch (Throwable t) {
   throw ExceptionUtil.unwrapThrowable(t);
  }
  //获取MapperMethod，并调用MapperMethod
  final MapperMethod mapperMethod = cachedMapperMethod(method);
  return mapperMethod.execute(sqlSession,args);
 }

 private MapperMethod cachedMapperMethod(Method method) {
  MapperMethod mapperMethod = methodCache.get(method);
  if (mapperMethod == null) {
   mapperMethod = new MapperMethod(mapperInterface,sqlSession.getConfiguration());
   methodCache.put(method,mapperMethod);
  }
  return mapperMethod;
 }

 @UsesJava7
 private Object invokeDefaultMethod(Object proxy,Object[] args)
   throws Throwable {
  final Constructor<MethodHandles.Lookup> constructor = MethodHandles.Lookup.class
    .getDeclaredConstructor(Class.class,int.class);
  if (!constructor.isAccessible()) {
   constructor.setAccessible(true);
  }
  final Class<?> declaringClass = method.getDeclaringClass();
  return constructor
    .newInstance(declaringClass,MethodHandles.Lookup.PRIVATE | MethodHandles.Lookup.PROTECTED
        | MethodHandles.Lookup.PACKAGE | MethodHandles.Lookup.PUBLIC)
    .unreflectSpecial(method,declaringClass).bindTo(proxy).invokeWithArguments(args);
 }

 /**
  * Backport of java.lang.reflect.Method#isDefault()
  */
 private boolean isDefaultMethod(Method method) {
  return ((method.getModifiers()
    & (Modifier.ABSTRACT | Modifier.PUBLIC | Modifier.STATIC)) == Modifier.PUBLIC)
    && method.getDeclaringClass().isInterface();
 }
}

所以这边需要进入MapperMethod的execute方法：

public Object execute(SqlSession sqlSession,Object[] args) {
  Object result;
  //判断是CRUD那种方法
  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 {
     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;
 }

然后，通过一层一层的调用，最终会来到doQuery方法， 这儿咱们就随便找个Excutor看看doQuery方法的实现吧，我这儿选择了SimpleExecutor:

public <E> List<E> doQuery(MappedStatement ms,Object parameter,RowBounds rowBounds,ResultHandler resultHandler,BoundSql boundSql) throws SQLException {
  Statement stmt = null;
  try {
   Configuration configuration = ms.getConfiguration();
   //内部封装了ParameterHandler和ResultSetHandler
   StatementHandler handler = configuration.newStatementHandler(wrapper,ms,parameter,rowBounds,resultHandler,boundSql);
   stmt = prepareStatement(handler,ms.getStatementLog());
   //StatementHandler封装了Statement,让 StatementHandler 去处理
   return handler.<E>query(stmt,resultHandler);
  } finally {
   closeStatement(stmt);
  }
 }

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