Mybatis Plugin插件

Plugin

首先mybatisPlugin是针对于mybatis四大组件（Statementhandler、resultsethandler、parameterHandler、executor）做增强操作的。
MyBatis 允许你在映射语句执行过程中的某一点进行拦截调用。默认情况下，MyBatis 允许使用插件来拦截的方法调用包括：
Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)
ParameterHandler (getParameterObject, setParameters)
ResultSetHandler (handleResultSets, handleOutputParameters)
StatementHandler (prepare, parameterize, batch, update, query)
通过 MyBatis 提供的强大机制，使用插件是非常简单的，只需实现 Interceptor 接口，并指定想要拦截的方法签名即可。
以pageHlper拦截器为例

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

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根据注解构建signatureMap

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;
  }

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key为自定义拦截器接口 ，参数时是方法参数
多个拦截器就会代理多次，对代理类再进行代理。

public interface Interceptor {
//执行目标方法
Object intercept(Invocation invocation) throws Throwable;
//执行pluginAll调用
Object plugin(Object target);
//xml配置的初始化参数值
void setProperties(Properties properties);
}
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我们要实现逻辑的方法是intercept
对于plugin方法，可以自定义一些场景

 @Override
    public Object plugin(Object target) {
        if (target instanceof Executor) {
            return Plugin.wrap(target, this);
        }
        return target;

    }
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例子

@Intercepts({@Signature(type = Executor.class, method = "query",
        args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})})
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  public Object intercept(Invocation invocation) throws Throwable {
        Object[] args = invocation.getArgs();
        MappedStatement ms = (MappedStatement) args[0];
        Object parameterObject = null;
        try {

             parameterObject = args[1];
        }catch (Exception e){
            System.out.println(e);
        }
        // id为执行的mapper方法的全路径名，如com.mapper.UserMapper
        String id = ms.getId();
        String sqlCommandType = ms.getSqlCommandType().toString();
        if (!sqlCommandType.equals(SqlCommandType.SELECT.toString())) {
            return invocation;
        }
        BoundSql boundSql = ms.getBoundSql(parameterObject);
        String origSql = boundSql.getSql();
        String newSql = getNewSql(id, origSql);
        if (newSql.equals(origSql)) {
            return invocation;
        }
        // 重新new一个查询语句对象
        BoundSql newBoundSql = new BoundSql(ms.getConfiguration(), newSql,
                boundSql.getParameterMappings(), boundSql.getParameterObject());

        // 把新的查询放到statement里
        MappedStatement newMs = newMappedStatement(ms, new BoundSqlSqlSource(newBoundSql));
        for (ParameterMapping mapping : boundSql.getParameterMappings()) {
            String prop = mapping.getProperty();
            if (boundSql.hasAdditionalParameter(prop)) {
                newBoundSql.setAdditionalParameter(prop, boundSql.getAdditionalParameter(prop));
            }
        }

        Object[] queryArgs = invocation.getArgs();
        queryArgs[0] = newMs;
        logger.info("由于数据权限，SQL被改写 originSql: {},newSql: {}", origSql, newSql);
        return invocation;
    }
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获取查询sql并改写

原理分析

再创建四大核心对象时会对对象做代理操作
例如
代理类Plugin

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;
  }


  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;
  }


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wrap方法会根据注解信息选择性的进行代理操作

package org.apache.ibatis.plugin;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

import org.apache.ibatis.reflection.ExceptionUtil;

/**
 * @author Clinton Begin
 */
public class Plugin implements InvocationHandler {


  @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);
    }
  }

}


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内部逻辑就是获取签名信息，执行时。匹配目标方法和注解中的方法参数，如果匹配上，那么执行拦截器interceptor。