概述

Plugin,意为插件,是mybatis为开发者提供的,对方法进行自定义编程的手段。其中用到了动态代理、反射方法,通过指定需要增强的对象与方法,进行程序编写。

核心类

主要涉及几个核心类:
Interceptor

Plugin

Intercepts

该增强功能的大致执行顺序为:

  1. 项目启动时,查询实现了
    Interceptor
    接口并且注册为Bean(或在xml文件中指定)的类,放入
    SqlSessionFactoryBean
    的Interceptor[]参数中,再由
    SqlSessionFactoryBean
    创建
    SqlSessionFactory
    的时候,将其放入
    Configuration
    参数中,留作后续调用

    // **注意interceptorsProvider,此为SpringBoot的configuration,会自动查询注册为Bean的Interceptor**
    public MybatisAutoConfiguration(MybatisProperties properties, ObjectProvider<Interceptor[]> interceptorsProvider,
          ObjectProvider<TypeHandler[]> typeHandlersProvider, ObjectProvider<LanguageDriver[]> languageDriversProvider,
          ResourceLoader resourceLoader, ObjectProvider<DatabaseIdProvider> databaseIdProvider,
          ObjectProvider<List<ConfigurationCustomizer>> configurationCustomizersProvider,
          ObjectProvider<List<SqlSessionFactoryBeanCustomizer>> sqlSessionFactoryBeanCustomizers) {
        this.properties = properties;
        this.interceptors = interceptorsProvider.getIfAvailable();
        this.typeHandlers = typeHandlersProvider.getIfAvailable();
        this.languageDrivers = languageDriversProvider.getIfAvailable();
        this.resourceLoader = resourceLoader;
        this.databaseIdProvider = databaseIdProvider.getIfAvailable();
        this.configurationCustomizers = configurationCustomizersProvider.getIfAvailable();
        this.sqlSessionFactoryBeanCustomizers = sqlSessionFactoryBeanCustomizers.getIfAvailable();
     }
     
     
    @Bean
    @ConditionalOnMissingBean
    public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
      SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
      factory.setDataSource(dataSource);
      .....
      // **注意此处,将interceptors放入factory中**
      if (!ObjectUtils.isEmpty(this.interceptors)) {
        factory.setPlugins(this.interceptors);
      }
    	.....
    }
    
    public class SqlSessionFactoryBean
    		protected SqlSessionFactory buildSqlSessionFactory() throws Exception {
    		....... 
    	    if (!isEmpty(this.typeAliases)) {
    	      Stream.of(this.typeAliases).forEach(typeAlias -> {
    	        targetConfiguration.getTypeAliasRegistry().registerAlias(typeAlias);
    	        LOGGER.debug(() -> "Registered type alias: '" + typeAlias + "'");
    	      });
    	    }
    	
    	    if (!isEmpty(this.plugins)) {
    	      Stream.of(this.plugins).forEach(plugin -> {
    	        targetConfiguration.addInterceptor(plugin);
    	        LOGGER.debug(() -> "Registered plugin: '" + plugin + "'");
    	      });
    	    }
    			.......
    	
    	    return this.sqlSessionFactoryBuilder.build(targetConfiguration);
    	  }
    }
    
  2. Configuration类在初始化
    ParameterHandler

    ResultSetHandler

    StatementHandler

    Executor
    四个类时,会对它们进行一次封装,封装内容即为用Interceptors注册插件功能,达到增强效果

    public class Configuration {
    
    	public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
        ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
        parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
        return parameterHandler;
      }
    
      public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
          ResultHandler resultHandler, BoundSql boundSql) {
        ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
        resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
        return resultSetHandler;
      }
    
      public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
        StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
        statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
        return statementHandler;
      }
      
      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;
      }
    
    }
    
  3. Configuration
    执行的pluginAll方法,内部是通过遍历Interceptor数组的plugin方法实现的。该方法入参和出参都是Object类型,所以可以认为它能为所有类型对象都进行增强封装。
    Interceptor
    内部调用了
    Plugin
    的wrap方法,对Object对象进行了封装。

    public class InterceptorChain {
    
      private final List<Interceptor> interceptors = new ArrayList<>();
    
      public Object pluginAll(Object target) {
        for (Interceptor interceptor : interceptors) {
          target = interceptor.plugin(target);
        }
        return target;
      }
    
      public void addInterceptor(Interceptor interceptor) {
        interceptors.add(interceptor);
      }
    
      public List<Interceptor> getInterceptors() {
        return Collections.unmodifiableList(interceptors);
      }
    
    }
    
    public interface Interceptor {
    
      Object intercept(Invocation invocation) throws Throwable;
    
      default Object plugin(Object target) {
        return Plugin.wrap(target, this);
      }
    
      default void setProperties(Properties properties) {
        // NOP
      }
    
    }
    
  4. Plugin
    方法实现了
    InvocationHandler
    动态代理类,并且wrap方法本身便是创建动态代理类。故Plugin类的职责有两项:


    1. 创建动态代理类,指定需要被代理(增强)的对象。此处为
      Executor

      Handler
      等。
    2. 指定被动态代理的对象,需要执行何种程序。重点关注
      invoke
      方法。

    public class Plugin implements InvocationHandler {
    
      private final Object target;
      private final Interceptor interceptor;
      private final Map<Class<?>, Set<Method>> signatureMap;
    
      private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {
        this.target = target;
        this.interceptor = interceptor;
        this.signatureMap = signatureMap;
      }
    
    	// **创建动态代理对象**
      public static Object wrap(Object target, Interceptor interceptor) {
        Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
        Class<?> type = target.getClass();
        Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
        if (interfaces.length > 0) {
          return Proxy.newProxyInstance(
              type.getClassLoader(),
              interfaces,
              new Plugin(target, interceptor, signatureMap));
        }
        return target;
      }
    
    	// **动态代理增强**
      @Override
      public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
          Set<Method> methods = signatureMap.get(method.getDeclaringClass());
          if (methods != null && methods.contains(method)) {
            return interceptor.intercept(new Invocation(target, method, args));
          }
          return method.invoke(target, args);
        } catch (Exception e) {
          throw ExceptionUtil.unwrapThrowable(e);
        }
      }
    	
    	// 省略getSignatureMap,getAllInterfaces方法
    	...
    }
    
  5. wrap方法执行时需要先通过interceptor获取signatureMap。
    Signature

    Intercepts
    注解中的value值注解,由于此value的返回值是数组,所以Signature会多个存在,最后解析出的结果便为signatureMap。

    Signature
    注解的作用为标注被动态代理的对象,具体的类型(class),具体的方法,方法具体的参数。只有特定类型和方法才会执行Interceptor方法。

    public class Plugin implements InvocationHandler {
    
      private final Object target;
      private final Interceptor interceptor;
      private final Map<Class<?>, Set<Method>> signatureMap;
    
      private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {
        this.target = target;
        this.interceptor = interceptor;
        this.signatureMap = signatureMap;
      }
    
      private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
        Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
        **// 通过此代码可发现,实现Interceptor的类必须添加Intercepts注解**
        if (interceptsAnnotation == null) {
          throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
        }
        Signature[] sigs = interceptsAnnotation.value();
        Map<Class<?>, Set<Method>> signatureMap = new HashMap<>();
        for (Signature sig : sigs) {
          Set<Method> methods = MapUtil.computeIfAbsent(signatureMap, sig.type(), k -> new HashSet<>());
          try {
    	      **// 通过Siganture的method与args,反射出Method对象,将其添加到map中
    	      // 作用是在执行动态代理invoke方法时,判断当前方法是否需要被interceptor执行**
            Method method = sig.type().getMethod(sig.method(), sig.args());
            methods.add(method);
          } catch (NoSuchMethodException e) {
            throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
          }
        }
        return signatureMap;
      }
    
    }
    
  6. 通过阅读源码可知,开发者需要自己实现
    Interceptor
    ,标记
    Intercepts
    注解,指定需要拦截的类、方法名,方法上的参数类型。并将Interceptor注册为Spring Bean。即可在interceptor方法中编写具体拦截代码。

实例

背景:在项目上为每一个需要插入至数据库中的实例对象,初始化id。

代码:

@Component
**// 拦截Executor类的update方法,该update方法会执行insert、update、delete操作**
@Intercepts(@Signature(type = Executor.class, method = "update", args = {MappedStatement.class, Object.class}))
public class MybatisUpdateInterceptor implements Interceptor {

	// 雪花算法id生成器
	@Autowired
	private IdGenerator idGenerator;

	@Override
	public Object intercept(Invocation invocation) throws Throwable {
		Method method = invocation.getMethod();
		MappedStatement ms = (MappedStatement) invocation.getArgs()[0];
		**// 判断是否为insert方法**
		if (ms.getSqlCommandType() != SqlCommandType.INSERT) {
			return invocation.proceed();
		}
		BaseEntity entity = (BaseEntity) invocation.getArgs()[1];
		if (entity.getId() == null) {
			entity.setId(idGenerator.generate());
		}
		return method.invoke(invocation.getTarget(), invocation.getArgs());
	}

}

总结

项目开发者可灵活运用plugin,为数据库操作进行增强。日常开发中也可借鉴此流程,通过动态代理方式设计拦截/增强手段。

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