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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：

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 interceptorsProvider,
      ObjectProvider typeHandlersProvider, ObjectProvider languageDriversProvider,
      ResourceLoader resourceLoader, ObjectProvider databaseIdProvider,
      ObjectProvider> configurationCustomizersProvider,
      ObjectProvider> 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);
	  }
}

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, param服务器托管eterObject, 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;
  }

}

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 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 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
  }

}

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：

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。
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, Set> signatureMap;

  private Plugin(Object target, Interceptor interceptor, Map, Set> 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, Set> 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 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
	...
}

wrap方法执行时需要先通过interceptor获取signatureMap。Signature是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, Set> signatureMap;

  private Plugin(Object target, Interceptor interceptor, Map, Set> signatureMap) {
    this.target = target;
    this.interceptor = interceptor;
    this.signatureMap = signatureMap;
  }

  private static Map, Set> 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, Set> signatureMap = new HashMap();
    for (Signature sig : sigs) {
      Set 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 me服务器托管thod on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
      }
    }
    return signatureMap;
  }

}

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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概述

核心类

实例

总结

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