Mybatis之插件原理
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鲁春利的工作笔记，好记性不如烂笔头
转载自：深入浅出Mybatis-插件原理
Mybatis采用责任链模式，通过动态代理组织多个拦截器（插件），通过这些拦截器可以改变Mybatis的默认行为（诸如SQL重写之类的），由于插件会深入到Mybatis的核心，因此在编写自己的插件前最好了解下它的原理，以便写出安全高效的插件。
代理链的生成
Mybatis支持对Executor、StatementHandler、PameterHandler和ResultSetHandler进行拦截，也就是说会对这4种对象进行代理。
下面以Executor为例。Mybatis在创建Executor对象时：
org.apache.ibatis.session.SqlSessionFactory.openSession()
org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSession()
org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromDataSource(execType, level, false) {
final Executor executor = configuration.newExecutor(tx, execType);
}org.apache.ibatis.session.Configuration
package org.apache.ibatis.session;
public class Configuration {
protected Environment environment;
protected String logPrefix;
protected Class <? extends Log> logImpl;
protected Class <? extends VFS> vfsImpl;
protected LocalCacheScope localCacheScope = LocalCacheScope.SESSION;
protected JdbcType jdbcTypeForNull = JdbcType.OTHER;
protected Set<String> lazyLoadTriggerMethods = new HashSet<String>(Arrays.asList(new String[] { "equals", "clone", "hashCode", "toString" }));
protected Integer defaultStatementTimeout;
protected Integer defaultFetchSize;
// 默认的Executor
protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;
protected AutoMappingBehavior autoMappingBehavior = AutoMappingBehavior.PARTIAL;
// InterceptorChain里保存了所有的拦截器，它在mybatis初始化的时候创建。
protected final InterceptorChain interceptorChain = new InterceptorChain();
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;
}
}org.apache.ibatis.plugin.InterceptorChain
package org.apache.ibatis.plugin;
/**
* @author Clinton Begin
*/
public class InterceptorChain {
private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
/**
* 每一个拦截器对目标类都进行一次代理
* @param target
* @return 层层代理后的对象
*/
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);
}
}下面以一个简单的例子来看看这个plugin方法里到底发生了什么。
package com.invicme.common.persistence.interceptor;
import java.sql.Connection;
import java.lang.reflect.Method;
import java.util.Properties;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.executor.statement.StatementHandler;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.plugin.Interceptor;
import org.apache.ibatis.plugin.Intercepts;
import org.apache.ibatis.plugin.Invocation;
import org.apache.ibatis.plugin.Plugin;
import org.apache.ibatis.plugin.Signature;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
*
* @author lucl
* @version 2017-01-09
*
* 示例的分页拦截器:
* <pre>
* @Intercepts用于表明当前的对象是一个Interceptor
* @Signature则表明要拦截的接口(type)、方法(method)以及对应的参数类型(args)
* type: Mybatis只支持对Executor、StatementHandler、PameterHandler和ResultSetHandler进行拦截，也就是说会对这4种对象进行代理。
* method: 这几个类的方法
* args: 这几个类的方法的参数
* </pre>
*
*/
@Intercepts(value = {
@Signature(type=Executor.class, method="query", args={MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
@Signature(type=StatementHandler.class, method="prepare", args={Connection.class})
})
public class SamplePagingInterceptor implements Interceptor {
private Logger logger = LoggerFactory.getLogger(getClass());
/**
* 拦截方法
* Invocation中定义了定义了一个proceed方法，其逻辑就是调用当前方法，所以如果在intercept中需要继续调用当前方法的话可以调用invocation的procced方法。
*/
@Override
public Object intercept(Invocation invocation) throws Throwable {
Object target = invocation.getTarget();
logger.info("target is {}", target.getClass().getName());
Method method = invocation.getMethod();
logger.info("method is {}", method.getName());
Object[] args = invocation.getArgs();
for (Object arg : args) {
logger.info("arg is {}", arg);
}
Object proceed = invocation.proceed();
logger.info("proceed is {}", proceed.getClass().getName());
return proceed;
}
/**
* 用于封装目标对象,该方法可以返回目标对象本身，也可以返回一个它的代理对象。
*/
@Override
public Object plugin(Object target) {
logger.info("target is {}", target.getClass().getName());
return Plugin.wrap(target, this);
}
/**
* 用于在Mybatis配置文件中指定一些属性
*/
@Override
public void setProperties(Properties properties) {
String value = properties.getProperty("sysuser");
logger.info("value is {}", value);
}
}每一个拦截器都必须实现上面的三个方法，其中：
1) Object intercept(Invocation invocation)是实现拦截逻辑的地方，内部要通过invocation.proceed()显式地推进责任链前进，也就是调用下一个拦截器拦截目标方法。
2) Object plugin(Object target)就是用当前这个拦截器生成对目标target的代理，实际是通过Plugin.wrap(target,this)来完成的，把目标target和拦截器this传给了包装函数。
3) setProperties(Properties properties)用于设置额外的参数，参数配置在拦截器的Properties节点里。
注解里描述的是指定拦截方法的签名 [type,method,args] （即对哪种对象的哪种方法进行拦截），它在拦截前用于决断。
代理链的生成
package com.invicme.common.aop.proxy;
import java.lang.reflect.Method;
import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.plugin.Interceptor;
import org.apache.ibatis.plugin.Intercepts;
import org.apache.ibatis.plugin.PluginException;
import org.apache.ibatis.plugin.Signature;
import org.apache.ibatis.session.Configuration;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import com.invicme.common.persistence.interceptor.SamplePagingInterceptor;
/**
*
* @author lucl
* @version 2017-01-18
*
* 测试Class对象
*
*/
public class MainDriver {
private static Logger logger = LoggerFactory.getLogger(MainDriver.class);
public static void main(String[] args) {
SamplePagingInterceptor interceptor = new SamplePagingInterceptor();
wrap(interceptor);
}
public static Object wrap (SamplePagingInterceptor interceptor) {
// 获取拦截器对应的@Intercepts注解
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
{
for (Iterator<Class<?>> it = signatureMap.keySet().iterator(); it.hasNext(); ) {
Class<?> clazz = it.next();
Set<Method> set = signatureMap.get(clazz);
logger.info("【Signature】clazz name is {}", clazz.getName());
for (Iterator<Method> itt = set.iterator(); itt.hasNext(); ) {
Method method = itt.next();
logger.info("\tmethod name is {}", method.getName());
Type[] genericParameterTypes = method.getGenericParameterTypes();
for (Type type : genericParameterTypes) {
logger.info("\t\tparam is {}", type);
}
}
}
}
logger.info("================================================================");
{
Configuration configuration = new Configuration();
Executor executor = configuration.newExecutor(null);
Class<?>[] interfaces = executor.getClass().getInterfaces();
for (Class<?> clazz : interfaces) {
logger.info("【Interface】clazz name is {}", clazz);
}
}
return null;
}
/**
* 获取拦截器对应的@Intercepts注解
* @param interceptor
* @return
*/
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<Class<?>, Set<Method>>();
for (Signature sig : sigs) {
// 处理重复值的问题
Set<Method> methods = signatureMap.get(sig.type());
if (methods == null) {
methods = new HashSet<Method>();
signatureMap.put(sig.type(), methods);
}
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;
}
}
从前面可以看出，每个拦截器的plugin方法是通过调用Plugin.wrap方法来实现的。代码如下：
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 {
private Object target;
private Interceptor interceptor;
private 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) {
// 从拦截器的注解中获取拦截的类名和方法信息
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
Class<?> type = target.getClass();
// 解析被拦截对象的所有接口（注意是接口）
Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
if (interfaces.length > 0) {
// 生成代理对象，Plugin对象为该代理对象的InvocationHandler
//（InvocationHandler属于java代理的一个重要概念，不熟悉的请参考相关概念）
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);
}
}
}这个Plugin类有三个属性：
private Object target; // 被代理的目标类
private Interceptor interceptor; // 对应的拦截器
private Map<Class<?>, Set<Method>> signatureMap; // 拦截器拦截的方法缓存
关于InvocationHandler请参阅：Java反射机制与动态代理
本文出自 “闷葫芦的世界” 博客，请务必保留此出处http://luchunli.blog.51cto.com/2368057/1892881
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