系列

• MyBatis攔截器原理介紹
• Mybatis攔截器改寫請(qǐng)求參數(shù)和結(jié)果
• Mybatis 插件兼容動(dòng)態(tài)SQL
• mybatis 參數(shù)解析流程附相關(guān)案例
• 再談mybatis執(zhí)行流程

Mybatis攔截器介紹

MyBatis 允許你在映射語句執(zhí)行過程中的某一點(diǎn)進(jìn)行攔截調(diào)用塑陵。默認(rèn)情況下，MyBatis 允許使用插件來攔截的方法調(diào)用包括：

• Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed) 攔截執(zhí)行器的方法
• ParameterHandler (getParameterObject, setParameters) 攔截參數(shù)的處理
• ResultSetHandler (handleResultSets, handleOutputParameters) 攔截結(jié)果集的處理
• StatementHandler (prepare, parameterize, batch, update, query) 攔截Sql語法和會(huì)話構(gòu)建的處理

mybatis執(zhí)行流程

• mybatis在執(zhí)行過程中按照Executor => StatementHandler => ParameterHandler => ResultSetHandler也颤。
• Executor在執(zhí)行過程中會(huì)創(chuàng)建StatementHandler，在創(chuàng)建StatementHandler過程中會(huì)創(chuàng)建 ParameterHandler和ResultSetHandler葫隙。

Mybatis攔截器的使用

---------定義攔截器
@Intercepts({
        @Signature(type = ParameterHandler.class, method = "setParameters", args = PreparedStatement.class),
        @Signature(type = Executor.class, method = "update", args = {MappedStatement.class, Object.class}),
        @Signature(type = Executor.class, method = "createCacheKey", args = {MappedStatement.class, Object.class, RowBounds.class, BoundSql.class})
})
public class ExamplePlugin implements Interceptor {
  private Properties properties = new Properties();
  public Object intercept(Invocation invocation) throws Throwable {
    // implement pre processing if need

    Object returnObject = invocation.proceed();

    // implement post processing if need
    return returnObject;
  }

  public void setProperties(Properties properties) {
    this.properties = properties;
  }
}

---------mybatis全局xml配置
<plugins>
    <plugin interceptor="org.format.mybatis.cache.interceptor.ExamplePlugin"></plugin>
</plugins>

自定義一個(gè)mybatis的攔截器步驟包含：

• 定義實(shí)現(xiàn)org.apache.ibatis.plugin.Interceptor接口的類娄帖。
• 在mybatis的全局配置xml中配置插件plugin。

public interface Executor {

  ResultHandler NO_RESULT_HANDLER = null;
  int update(MappedStatement ms, Object parameter) throws SQLException;
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;
  <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;
  List<BatchResult> flushStatements() throws SQLException;
  void commit(boolean required) throws SQLException;
  void rollback(boolean required) throws SQLException;
  CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);
  boolean isCached(MappedStatement ms, CacheKey key);
  void clearLocalCache();
  void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);
  Transaction getTransaction();
  void close(boolean forceRollback);
  boolean isClosed();
  void setExecutorWrapper(Executor executor);
}


public interface ParameterHandler {

  Object getParameterObject();
  void setParameters(PreparedStatement ps) throws SQLException;
}


public interface ResultSetHandler {

  <E> List<E> handleResultSets(Statement stmt) throws SQLException;
  <E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException;
  void handleOutputParameters(CallableStatement cs) throws SQLException;
}


public interface StatementHandler {

  Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException;
  void parameterize(Statement statement) throws SQLException;
  void batch(Statement statement) throws SQLException;
  int update(Statement statement) hrows SQLException;
  <E> List<E> query(Statement statement, ResultHandler resultHandler) hrows SQLException;
  <E> Cursor<E> queryCursor(Statement statement) hrows SQLException;
  BoundSql getBoundSql();
  ParameterHandler getParameterHandler();
}

攔截點(diǎn)說明：

• 不同攔截點(diǎn)能夠支持?jǐn)r截的方法麦撵。
• 攔截方法的type上遥、method搏屑、args等都取自上述方法。

Mybatis攔截器原理

攔截器的原理核心的內(nèi)容在于攔截器配置解析粉楚、攔截器的職責(zé)鏈構(gòu)造辣恋、攔截器的執(zhí)行，串聯(lián)上述功能后就能傳統(tǒng)整體的流程模软。

攔截器解析

private void parseConfiguration(XNode root) {
    try {
      // issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      loadCustomLogImpl(settings);
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));  // 解析插件相關(guān)的參數(shù)
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }


  private void pluginElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) { // 解析生成攔截器
        String interceptor = child.getStringAttribute("interceptor");
        Properties properties = child.getChildrenAsProperties();
        Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).getDeclaredConstructor().newInstance();
        interceptorInstance.setProperties(properties);
        configuration.addInterceptor(interceptorInstance); // 保存攔截器
      }
    }
  }


public class InterceptorChain {

  private final List<Interceptor> interceptors = new ArrayList<>(); // 全局變量保存所有的攔截器

  // 織入攔截器
  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

  // 負(fù)責(zé)保存所有的攔截器
  public void addInterceptor(Interceptor interceptor) {
    interceptors.add(interceptor);
  }

  // 獲取所有攔截器
  public List<Interceptor> getInterceptors() {
    return Collections.unmodifiableList(interceptors);
  }

}

攔截器解析說明：

• 負(fù)責(zé)解析XML配置文件解析所有的攔截器對(duì)象保存到攔截器鏈InterceptorChain 伟骨。
• InterceptorChain通過pluginAll植入攔截器。

攔截器織入

public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
    // 插入攔截器
    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);
    // 插入攔截器
    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);
    // 插入攔截器
    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);
    }

    // 插入攔截器
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

攔截器織入說明：

• 針對(duì)ParameterHandler 燃异、ResultSetHandler 携狭、StatementHandler 、Executor 的攔截器的植入都是通過interceptorChain.pluginAll來實(shí)現(xiàn)回俐。

public class InterceptorChain {

  private final List<Interceptor> interceptors = new ArrayList<>();

  public Object pluginAll(Object target) {
    // 遍歷所有的攔截器負(fù)責(zé)進(jìn)行植入
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }
}

public interface Interceptor {

  Object intercept(Invocation invocation) throws Throwable;
  // 通過wrap織入單個(gè)攔截器
  default Object plugin(Object target) {
    return Plugin.wrap(target, this);
  }

  default void setProperties(Properties properties) {
    // NOP
  }

}

public class Plugin implements InvocationHandler {

  public static Object wrap(Object target, Interceptor interceptor) {
    // 解析攔截器的注解獲取攔截的攔截點(diǎn)和對(duì)應(yīng)的攔截方法
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    Class<?> type = target.getClass();

    // 獲取被攔截對(duì)應(yīng)的信息構(gòu)建JDK的動(dòng)態(tài)代理
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    if (interfaces.length > 0) {
      // 針對(duì)注解指定的interface進(jìn)行攔截
      return Proxy.newProxyInstance(
          type.getClassLoader(),
          interfaces,
          new Plugin(target, interceptor, signatureMap));
    }
    return target;
  }
}

攔截器織入說明：

• 攔截器織入依賴于JDK自帶的Proxy.newProxyInstance動(dòng)態(tài)代理來實(shí)現(xiàn)逛腿。
• 返回的對(duì)象是個(gè)Plugin對(duì)象的動(dòng)態(tài)代理。

  private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
    // 1仅颇、獲取所有攔截的Intercepts注解
    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());
    }
    // 2单默、獲取注解的內(nèi)容
    Signature[] sigs = interceptsAnnotation.value();
    Map<Class<?>, Set<Method>> signatureMap = new HashMap<>();
    // 3、遍歷注解按照type + method維度進(jìn)行構(gòu)建
    for (Signature sig : sigs) {
      Set<Method> methods = MapUtil.computeIfAbsent(signatureMap, 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) {
      // 獲取攔截點(diǎn)對(duì)應(yīng)的Interfaces并且在攔截點(diǎn)注解配置的的interface用以構(gòu)建動(dòng)態(tài)代理
      for (Class<?> c : type.getInterfaces()) {
        if (signatureMap.containsKey(c)) {
          interfaces.add(c);
        }
      }
      type = type.getSuperclass();
    }
    return interfaces.toArray(new Class<?>[0]);
  }

攔截器織入說明：

• 通過解析注解@Intercepts注解@Signature來構(gòu)建class + method兩個(gè)維度的map對(duì)象灵莲。
• 以@Signature(type = ParameterHandler.class, method = "setParameters", args = PreparedStatement.class)為例，會(huì)構(gòu)建key為ParameterHandler殴俱，value為setParameters的map

攔截器執(zhí)行

public class Plugin implements InvocationHandler {

  private final Object target;
  private final Interceptor interceptor;
  private final Map<Class<?>, Set<Method>> signatureMap;

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      // 判斷是否是被攔截的方法政冻，如果是就通過攔截器進(jìn)行處理
      Set<Method> methods = signatureMap.get(method.getDeclaringClass());
      if (methods != null && methods.contains(method)) {
        // 由攔截器interceptor負(fù)責(zé)處理
        return interceptor.intercept(new Invocation(target, method, args));
      }
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }
}

JDK動(dòng)態(tài)代理

public class Main {
    public static void main(String[] args) throws Exception {
        
        final Dog dog = new Dog();

        IDog proxy = (IDog)Proxy.newProxyInstance(Dog.class.getClassLoader(), Dog.class.getInterfaces(), new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

                return method.invoke(dog, args);
            }
        });

        proxy.eat();

        Thread.sleep(100000);
    }
}


-------------------------對(duì)應(yīng)生成的動(dòng)態(tài)代碼

package com.sun.proxy;

import com.sunboy.IDog;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class $Proxy0 extends Proxy implements IDog {
    private static Method m1;
    private static Method m3;
    private static Method m2;
    private static Method m0;

    public $Proxy0(InvocationHandler invocationHandler) {
        super(invocationHandler);
    }

    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m3 = Class.forName("com.sunboy.IDog").getMethod("eat", new Class[0]);
            m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
            m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
            return;
        }
        catch (NoSuchMethodException noSuchMethodException) {
            throw new NoSuchMethodError(noSuchMethodException.getMessage());
        }
        catch (ClassNotFoundException classNotFoundException) {
            throw new NoClassDefFoundError(classNotFoundException.getMessage());
        }
    }

    public final boolean equals(Object object) {
        try {
            return (Boolean)this.h.invoke(this, m1, new Object[]{object});
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final String toString() {
        try {
            return (String)this.h.invoke(this, m2, null);
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final int hashCode() {
        try {
            return (Integer)this.h.invoke(this, m0, null);
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final void eat() {
        try {
            this.h.invoke(this, m3, null);
            return;
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }
}

• JDK動(dòng)態(tài)代理生成的代理類如$Proxy0枚抵，通過arthas去反編譯類。
• JDK動(dòng)態(tài)代理通過反射獲取Method對(duì)象用以后續(xù)的執(zhí)行明场。