SSM项目中Mapper的加载和动态代理的实现原理

一、bean解析阶段

MapperScannerConfigurer xml配置

MapperScannerConfigurer类主要成员变量如下：

  private String basePackage;
  private boolean addToConfig = true;
  private SqlSessionFactory sqlSessionFactory;
  private SqlSessionTemplate sqlSessionTemplate;
  private String sqlSessionFactoryBeanName;
  private String sqlSessionTemplateBeanName;
  private Class annotationClass;
  private Class markerInterface;
  private ApplicationContext applicationContext;
  private String beanName;
  private boolean processPropertyPlaceHolders;
  private BeanNameGenerator nameGenerator;

spring加载配置文件，成员变量spring为其装配，该类实现BeanDefinitionRegistryPostProcessor(extends BeanFactoryPostProcessor) 接口，postProcessBeanDefinitionRegistry会被Spring先于postProcessBeanFactory这个方法执行，
具体执行方法如下：

@Override
  public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
    if (this.processPropertyPlaceHolders) {
      processPropertyPlaceHolders();
    }

    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
    scanner.setAddToConfig(this.addToConfig);
    scanner.setAnnotationClass(this.annotationClass);
    scanner.setMarkerInterface(this.markerInterface);
    scanner.setSqlSessionFactory(this.sqlSessionFactory);
    scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
    scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName);
    scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName);
    scanner.setResourceLoader(this.applicationContext);
    scanner.setBeanNameGenerator(this.nameGenerator);
    scanner.registerFilters();
    scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
  }

processPropertyPlaceHolders默认不设置为false，由ClassPathMapperScanner进行扫描，调用其父类ClassPathBeanDefinitionScanner的scan方法，

public int scan(String... basePackages) {
   int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
    this.doScan(basePackages);
    if(this.includeAnnotationConfig) {
        AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
    }

    return this.registry.getBeanDefinitionCount() - beanCountAtScanStart;
}

this.doScan调用的是ClassPathMapperScanner的重写方法：

/**
   * Calls the parent search that will search and register all the candidates.
   * Then the registered objects are post processed to set them as
   * MapperFactoryBeans
   */
  @Override
  public Set doScan(String... basePackages) {
	    Set beanDefinitions = super.doScan(basePackages);
	
	    if (beanDefinitions.isEmpty()) {
	      logger.warn("No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.");
	    } else {
	      processBeanDefinitions(beanDefinitions);
	    }
	
	    return beanDefinitions;
  }

方法内调用父类ClassPathBeanDefinitionScanner的doScan方法解析目标包下的所有bean并注册到容器中：

protected Set doScan(String... basePackages) {
        Assert.notEmpty(basePackages, "At least one base package must be specified");
        Set beanDefinitions = new LinkedHashSet();
        String[] var3 = basePackages;
        int var4 = basePackages.length;

        for(int var5 = 0; var5 < var4; ++var5) {
            String basePackage = var3[var5];
            Set candidates = this.findCandidateComponents(basePackage);
            Iterator var8 = candidates.iterator();

            while(var8.hasNext()) {
                BeanDefinition candidate = (BeanDefinition)var8.next();
                ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
                candidate.setScope(scopeMetadata.getScopeName());
                String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
                if(candidate instanceof AbstractBeanDefinition) {
                    this.postProcessBeanDefinition((AbstractBeanDefinition)candidate, beanName);
                }

                if(candidate instanceof AnnotatedBeanDefinition) {
                    AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition)candidate);
                }

                if(this.checkCandidate(beanName, candidate)) {
                    BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
                    definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
                    beanDefinitions.add(definitionHolder);
                    this.registerBeanDefinition(definitionHolder, this.registry);
                }
            }
        }

        return beanDefinitions;
    }

最后ClassPathMapperScanner调用processBeanDefinitions方法对注册到容器中的beanDefinitions进行二次封装（beanClass=mapperFactoryBean和sqlSessionFactory等配置）

二、bean实例化阶段

上面提到了封装mapper bean时beanClass指定的是MapperFactoryBean

实例化调用MapperFactoryBean的getObject方法

/**
   * {@inheritDoc}
   */
  @Override
  public T getObject() throws Exception {
    return getSqlSession().getMapper(this.mapperInterface);
  }

这里的getSqlSession().getMapper(this.mapperInterface);方法默认是DefaultSqlSession调用

@Override
  public  T getMapper(Class type) {
    return configuration.getMapper(type, this);
  }

configuration下再调用MapperRegistry的getMapper方法：

@SuppressWarnings("unchecked")
  public  T getMapper(Class type, SqlSession sqlSession) {
    final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) 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);
    }
  }

knowMappers中封装的bean实际是sqlSessionFactory在spring进行装配的时候预先加载好的，

        
        
        	 
                classpath*:/config/mappers/system/*.xml
                classpath*:/config/mappers/*.xml
            
        
		
    

到此，确定mapperProxyFactory.newInstance(sqlSession) mybatis动态代理的实现路径
动态代理实现如下:

/**
 * @author Lasse Voss
 */
public class MapperProxyFactory {

  private final Class mapperInterface;
  private final Map methodCache = new ConcurrentHashMap();

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

  public Class getMapperInterface() {
    return mapperInterface;
  }

  public Map getMethodCache() {
    return methodCache;
  }

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

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

}

mapper方法的执行实际上是调用了MapperMethod的execute方法

MapperMethod是整个代理机制的核心类，对Sqlsession当中的操作进行了封装，类里面有俩个成员：SqlCommand类和MethodSignature类。从名字上我们大概的能想到一个可能跟SQL语句有关系，一个可能跟要执行的方法有关系。事实也是如此。

上面代码使用一个内部类SqlCommand来封装底层的增删改查操作，确切来讲这一部分的内容跟XxxMapper的XML配置文件里面的select节点、delete节点等有关。我们都会知道节点上有id属性值。那么MyBatis框架会把每一个节点(如：select节点、delete节点）生成一个MappedStatement类。要找到MappedStatement类就必须通过id来获得。有一个细节要注意：代码用到的id = 当前接口类 + XML文件的节点的ID属性。其源码如下：

//一个内部类 封装了具体执行的动作

public static class SqlCommand {
    //xml标签的id
    private final String name;
    //insert update delete select的具体类型
    private final SqlCommandType type;
    public SqlCommand(Configuration configuration, Class mapperInterface, Method method) throws BindingException {
      //拿到全名 比如 org.mybatis.example.BlogMapper.selectBlog
      String statementName = mapperInterface.getName() + "." + method.getName();
      MappedStatement ms = null;
      //获取MappedStatement对象 这个对象封装了XML当中一个标签的所有信息 比如下面
      //
      //select * from Blog where id = #{id}
      //
      if (configuration.hasStatement(statementName)) {
        ms = configuration.getMappedStatement(statementName);
      } else if (!mapperInterface.equals(method.getDeclaringClass().getName())) { // 这里是一个BUG
        String parentStatementName = method.getDeclaringClass().getName() + "." + method.getName();
        if (configuration.hasStatement(parentStatementName)) {
          ms = configuration.getMappedStatement(parentStatementName);
        }
      }
      //为空抛出异常
      if (ms == null) {
        throw new BindingException("Invalid bound statement (not found): " + statementName);
      }
      name = ms.getId();
      type = ms.getSqlCommandType();
      //判断SQL标签类型 未知就抛异常
      if (type == SqlCommandType.UNKNOWN) {
        throw new BindingException("Unknown execution method for: " + name);
      }
    }
    public String getName() {
      return name;
    }
    public SqlCommandType getType() {
      return type;
    }
  }
  //内部类 封装了接口当中方法的 参数类型 返回值类型 等信息
  public static class MethodSignature {
    //是否返回多调结果
    private final boolean returnsMany;
    //返回值是否是MAP
    private final boolean returnsMap;
    //返回值是否是VOID
    private final boolean returnsVoid;
    //返回值类型
    private final Class returnType;
    //mapKey
    private final String mapKey;
    //resultHandler类型参数的位置
    private final Integer resultHandlerIndex;
    //rowBound类型参数的位置
    private final Integer rowBoundsIndex;
    //用来存放参数信息
    private final SortedMap params;
    //是否存在命名参数
    private final boolean hasNamedParameters;
    //在这里对上面的属性进行初始化 就不一一详细说明了 具体实现细节可以看下面的代码。
    public MethodSignature(Configuration configuration, Method method) throws BindingException {
      this.returnType = method.getReturnType();
      this.returnsVoid = void.class.equals(this.returnType);
      this.returnsMany = (configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray());
      this.mapKey = getMapKey(method);
      this.returnsMap = (this.mapKey != null);
      this.hasNamedParameters = hasNamedParams(method);
      this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
      this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
      this.params = Collections.unmodifiableSortedMap(getParams(method, this.hasNamedParameters));
    }
  ...

有什么不对的地方，欢迎指教，谢谢