美团一面:你既然写过Mybatis插件,说说它底层是怎么加载一个自定义插件的

大多数框架,都支持插件,用户可通过编写插件来自行扩展功能,Mybatis也不例外。

我们从插件配置、插件编写、插件运行原理、插件注册与执行拦截的时机、初始化插件、分页插件的原理等六个方面展开阐述。

1. 插件配置

Mybatis的插件配置在configuration内部,初始化时,会读取这些插件,保存于Configuration对象的InterceptorChain中。整理了一份272页Mybatis学习笔记




    
    
      
    
  


public class Configuration {
    protected final InterceptorChain interceptorChain = new InterceptorChain();
}

org.apache.ibatis.plugin.InterceptorChain.java源码。

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);
  }
}

上面的for循环代表了只要是插件,都会以责任链的方式逐一执行(别指望它能跳过某个节点),所谓插件,其实就类似于拦截器。

2. 如何编写一个插件

插件必须实现org.apache.ibatis.plugin.Interceptor接口。

public interface Interceptor {

  Object intercept(Invocation invocation) throws Throwable;

  Object plugin(Object target);

  void setProperties(Properties properties);

}

intercept()方法:执行拦截内容的地方,比如想收点保护费。由plugin()方法触发,interceptor.plugin(target)足以证明。

plugin()方法:决定是否触发intercept()方法。

setProperties()方法:给自定义的拦截器传递xml配置的属性参数。

下面自定义一个拦截器:

@Intercepts({
    @Signature(type = Executor.class, method = "query",
    args = { MappedStatement.class, Object.class,
        RowBounds.class, ResultHandler.class }),
    @Signature(type = Executor.class, method = "close",     
    args = { boolean.class }) })
public class MyBatisInterceptor implements Interceptor {

  private Integer value;

  @Override
  public Object intercept(Invocation invocation) throws Throwable {
    return invocation.proceed();
  }

  @Override
  public Object plugin(Object target) {
    System.out.println(value);
        // Plugin类是插件的核心类,用于给target创建一个JDK的动态代理对象,触发intercept()方法
    return Plugin.wrap(target, this);
  }

  @Override
  public void setProperties(Properties properties) {
    value = Integer.valueOf((String) properties.get("value"));
  }

}

面对上面的代码,我们需要解决两个疑问:

1.为什么要写Annotation注解?注解都是什么含义?

答: Mybatis规定插件必须编写Annotation注解,是必须,而不是可选。

@Intercepts注解:装载一个@Signature列表,一个@Signature其实就是一个需要拦截的方法封装。那么,一个拦截器要拦截多个方法,自然就是一个@Signature列表。

type = Executor.class, 
method = "query", 
args = { MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class }

解释: 要拦截Executor接口内的query()方法,参数类型为args列表。

2. Plugin.wrap(target, this)是干什么的?

答: 使用JDK的动态代理,给target对象创建一个delegate代理对象,以此来实现方法拦截和增强功能,它会回调intercept()方法。

org.apache.ibatis.plugin.Plugin.java源码:

public class Plugin implements InvocationHandler {

  private Object target;
  private Interceptor interceptor;
  private Map, Set> signatureMap;

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

  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) {
      // 创建JDK动态代理对象
      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 methods = signatureMap.get(method.getDeclaringClass());
      // 判断是否是需要拦截的方法(很重要)
      if (methods != null && methods.contains(method)) {
        // 回调intercept()方法
        return interceptor.intercept(new Invocation(target, method, args));
      }
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }
//...
}

Map, Set> signatureMap:缓存需拦截对象的反射结果,避免多次反射,即target的反射结果。

所以,我们不要动不动就说反射性能很差,那是因为你没有像Mybatis一样去缓存一个对象的反射结果。

判断是否是需要拦截的方法,这句注释很重要,一旦忽略了,都不知道Mybatis是怎么判断是否执行拦截内容的,要记住。

3. Mybatis可以拦截哪些接口对象?

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); // 1
    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); // 2
    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); // 3
    return statementHandler;
  }

  public Executor newExecutor(Transaction transaction) {
    return newExecutor(transaction, defaultExecutorType);
  }

  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); // 4
    return executor;
  }
//...
}

Mybatis只能拦截ParameterHandler、ResultSetHandler、StatementHandler、Executor共4个接口对象内的方法。

重新审视interceptorChain.pluginAll()方法:该方法在创建上述4个接口对象时调用,其含义为给这些接口对象注册拦截器功能,注意是注册,而不是执行拦截。

拦截器执行时机:plugin()方法注册拦截器后,那么,在执行上述4个接口对象内的具体方法时,就会自动触发拦截器的执行,也就是插件的执行。

所以,一定要分清,何时注册,何时执行。切不可认为pluginAll()或plugin()就是执行,它只是注册。

4. Invocation

public class Invocation {
  private Object target;
  private Method method;
  private Object[] args;
}

intercept(Invocation invocation)方法的参数Invocation ,我相信你一定可以看得懂,不解释。

5. 初始化插件源码解析

org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XNode)方法部分源码。

pluginElement(root.evalNode("plugins"));

 private void pluginElement(XNode parent) throws Exception {
  if (parent != null) {
    for (XNode child : parent.getChildren()) {
      String interceptor = child.getStringAttribute("interceptor");
      Properties properties = child.getChildrenAsProperties();
      Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
      // 这里展示了setProperties()方法的调用时机
      interceptorInstance.setProperties(properties);
      configuration.addInterceptor(interceptorInstance);
    }
  }
}

对于Mybatis,它并不区分是何种拦截器接口,所有的插件都是Interceptor,Mybatis完全依靠Annotation去标识对谁进行拦截,所以,具备接口一致性。

6. 分页插件原理

由于Mybatis采用的是逻辑分页,而非物理分页,那么,市场上就出现了可以实现物理分页的Mybatis的分页插件。

要实现物理分页,就需要对String sql进行拦截并增强,Mybatis通过BoundSql对象存储String sql,而BoundSql则由StatementHandler对象获取。整理了一份272页Mybatis学习笔记

public interface StatementHandler {
     List query(Statement statement, ResultHandler resultHandler) throws SQLException;
    BoundSql getBoundSql();
}

public class BoundSql {
   public String getSql() {
    return sql;
  }
}

因此,就需要编写一个针对StatementHandler的query方法拦截器,然后获取到sql,对sql进行重写增强。

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