Mybatis（八）：插件机制

发布日期: 2021-08-26

插件机制

介绍

微内核架构，也就是插件化架构，是一种面向功能进行拆分的可扩展架构。指的是软件的内核相对较小，主要功能和业务逻辑都通过插件实现。插件分为两类，一类是对系统的补充，一类是对系统默认功能的自定义修改。Mybatis的插件机制就属于第二类，实现了拦截器的功能，可拦截特定对象进行自定义处理。

拦截的目标对象

Mybatis拦截的对象就是其最重要的4大对象，创建这4大对象之后都对其进行了代理包装，之后代理类就代替了这4大对象进行操作。

ParameterHandler

public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
  ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
  parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
  return parameterHandler;
}

ResultSetHandler

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;
}

StatementHandler

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;
}

Executor

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;
}

实现机制

Mybatis编码上采用责任链模式，通过动态代理包装组织插件，通过这些插件就可以自定义Mybatis的行为

关键类

Interceptor、@Intercepts、@Signature

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
  }

}

每个插件都需要实现Interceptor接口，实现其intercept方法，该方法执行自定义逻辑后返回执行结果

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Intercepts {
  /**
   * Returns method signatures to intercept.
   *
   * @return method signatures
   */
  Signature[] value();
}

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({})
public @interface Signature {
  /**
   * Returns the java type.
   *
   * @return the java type
   */
  Class<?> type();

  /**
   * Returns the method name.
   *
   * @return the method name
   */
  String method();

  /**
   * Returns java types for method argument.
   * @return java types for method argument
   */
  Class<?>[] args();
}

@Intercepts注解是个组合注解，包装@Signature注解，它们表示拦截器需要拦截的目标

举例1：

@Test
void mapPluginShouldInterceptGet() {
  Map map = new HashMap();
  map = (Map) new AlwaysMapPlugin().plugin(map);
  assertEquals("Always", map.get("Anything"));
}

@Test
void shouldNotInterceptToString() {
  Map map = new HashMap();
  map = (Map) new AlwaysMapPlugin().plugin(map);
  assertNotEquals("Always", map.toString());
}

@Intercepts({
    @Signature(type = Map.class, method = "get", args = {Object.class})})
public static class AlwaysMapPlugin implements Interceptor {
  @Override
  public Object intercept(Invocation invocation) {
    return "Always";
  }

}

上面的例子中AlwaysMapPlugin实现了Interceptor接口，指定拦截Map类的参数为Object的get方法，并实现了intercept方法。通过plugin方法包装HashMap类，当调用Map的get方法时，拦截器的intercept方法就被执行了，而调用其它方法则不会执行intercept方法。

InterceptorChain

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);
  }

}

拦截器责任链是Configuration的一个属性，保存着所有的拦截器集合interceptors，最重要的方法就是pluginAll，遍历所有的拦截器，调用它们的plugin方法进行层层包装，其实就是一个代理又包着一个代理，像洋葱一样。

Plugin

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;
  }

  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;
  }

  @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);
    }
  }

  private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
    Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
    // issue #251
    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 = signatureMap.computeIfAbsent(sig.type(), k -> new HashSet<>());
      try {
        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;
  }

  private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
    Set<Class<?>> interfaces = new HashSet<>();
    while (type != null) {
      for (Class<?> c : type.getInterfaces()) {
        if (signatureMap.containsKey(c)) {
          interfaces.add(c);
        }
      }
      type = type.getSuperclass();
    }
    return interfaces.toArray(new Class<?>[0]);
  }

}

动态代理增强类是InvocationHandler执行器，通过InvocationHandler的invoke方法实现逻辑增强。Plugin类实现了InvocationHandler接口。其最重要的方法就是wrap、invoke方法。wrap方法用来包装创建代理类，invoke方法执行拦截器的intercept方法。