Mybatis懒加载的实现
因为通过javassist和cglib代理实现的，所以说到底最主要的就是JavasisstProxyFactory类中的invoke方法和里面的load方法。
其实读一读，里面的逻辑就是跟配置中定义的规则一样的
因为github上的mybatis中文版中这部分注释比较少，所以从网上寻找博客，截取了代码注释片段记录下。
JavasisstProxyFactory
publicclassJavassistProxyFactoryimplementsorg.apache.ibatis.executor.loader.ProxyFactory{
/**
*接口实现
*@paramtarget目标结果对象
*@paramlazyLoader延迟加载对象
*@paramconfiguration配置
*@paramobjectFactory对象工厂
*@paramconstructorArgTypes构造参数类型
*@paramconstructorArgs构造参数值
*@return
*/
@Override
publicObjectcreateProxy(Objecttarget,ResultLoaderMaplazyLoader,Configurationconfiguration,ObjectFactoryobjectFactory,List<Class<?>>constructorArgTypes,List<Object>constructorArgs){
returnEnhancedResultObjectProxyImpl.createProxy(target,lazyLoader,configuration,objectFactory,constructorArgTypes,constructorArgs);
}
//省略...
/**
*代理对象实现，核心逻辑执行
*/
privatestaticclassEnhancedResultObjectProxyImplimplementsMethodHandler{
/**
*创建代理对象
*@paramtype
*@paramcallback
*@paramconstructorArgTypes
*@paramconstructorArgs
*@return
*/
staticObjectcrateProxy(Class<?>type,MethodHandlercallback,List<Class<?>>constructorArgTypes,List<Object>constructorArgs){
ProxyFactoryenhancer=newProxyFactory();
enhancer.setSuperclass(type);
try{
//通过获取对象方法，判断是否存在该方法
type.getDeclaredMethod(WRITE_REPLACE_METHOD);
//ObjectOutputStreamwillcallwriteReplaceofobjectsreturnedbywriteReplace
if(log.isDebugEnabled()){
log.debug(WRITE_REPLACE_METHOD+"methodwasfoundonbean"+type+",makesureitreturnsthis");
}
}catch(NoSuchMethodExceptione){
//没找到该方法，实现接口
enhancer.setInterfaces(newClass[]{WriteReplaceInterface.class});
}catch(SecurityExceptione){
//nothingtodohere
}
Objectenhanced;
Class<?>[]typesArray=constructorArgTypes.toArray(newClass[constructorArgTypes.size()]);
Object[]valuesArray=constructorArgs.toArray(newObject[constructorArgs.size()]);
try{
//创建新的代理对象
enhanced=enhancer.create(typesArray,valuesArray);
}catch(Exceptione){
thrownewExecutorException("Errorcreatinglazyproxy.Cause:"+e,e);
}
//设置代理执行器
((Proxy)enhanced).setHandler(callback);
returnenhanced;
}
/**
*代理对象执行
*@paramenhanced原对象
*@parammethod原对象方法
*@parammethodProxy代理方法
*@paramargs方法参数
*@return
*@throwsThrowable
*/
@Override
publicObjectinvoke(Objectenhanced,Methodmethod,MethodmethodProxy,Object[]args)throwsThrowable{
finalStringmethodName=method.getName();
try{
synchronized(lazyLoader){
if(WRITE_REPLACE_METHOD.equals(methodName)){
//忽略暂未找到具体作用
Objectoriginal;
if(constructorArgTypes.isEmpty()){
original=objectFactory.create(type);
}else{
original=objectFactory.create(type,constructorArgTypes,constructorArgs);
}
PropertyCopier.copyBeanProperties(type,enhanced,original);
if(lazyLoader.size()>0){
returnnewJavassistSerialStateHolder(original,lazyLoader.getProperties(),objectFactory,constructorArgTypes,constructorArgs);
}else{
returnoriginal;
}
}else{
//延迟加载数量大于0
if(lazyLoader.size()>0&&!FINALIZE_METHOD.equals(methodName)){
//aggressive一次加载性所有需要要延迟加载属性或者包含触发延迟加载方法
if(aggressive||lazyLoadTriggerMethods.contains(methodName)){
log.debug("==>lazelodtriggermethod:"+methodName+",proxymethod:"+methodProxy.getName()+"class:"+enhanced.getClass());
//一次全部加载
lazyLoader.loadAll();
}elseif(PropertyNamer.isSetter(methodName)){
//判断是否为set方法，set方法不需要延迟加载
finalStringproperty=PropertyNamer.methodToProperty(methodName);
lazyLoader.remove(property);
}elseif(PropertyNamer.isGetter(methodName)){
finalStringproperty=PropertyNamer.methodToProperty(methodName);
if(lazyLoader.hasLoader(property)){
//延迟加载单个属性
lazyLoader.load(property);
log.debug("loadone:"+methodName);
}
}
}
}
}
returnmethodProxy.invoke(enhanced,args);
}catch(Throwablet){
throwExceptionUtil.unwrapThrowable(t);
}
}
}
load方法
/**
*执行懒加载查询，获取数据并且set到userObject中返回
*@paramuserObject
*@throwsSQLException
*/
publicvoidload(finalObjectuserObject)throwsSQLException{
//合法性校验
if(this.metaResultObject==null||this.resultLoader==null){
if(this.mappedParameter==null){
thrownewExecutorException("Property["+this.property+"]cannotbeloadedbecause"
+"requiredparameterofmappedstatement["
+this.mappedStatement+"]isnotserializable.");
}
//获取mappedstatement并且校验
finalConfigurationconfig=this.getConfiguration();
finalMappedStatementms=config.getMappedStatement(this.mappedStatement);
if(ms==null){
thrownewExecutorException("Cannotlazyloadproperty["+this.property
+"]ofdeserializedobject["+userObject.getClass()
+"]becauseconfigurationdoesnotcontainstatement["
+this.mappedStatement+"]");
}
//使用userObject构建metaobject,并且重新构建resultloader对象
this.metaResultObject=config.newMetaObject(userObject);
this.resultLoader=newResultLoader(config,newClosedExecutor(),ms,this.mappedParameter,
metaResultObject.getSetterType(this.property),null,null);
}
/*Weareusinganewexecutorbecausewemaybe(andlikelyare)onanewthread
*andexecutorsaren'tthreadsafe.(Isthissufficient?)
*
*Abetterapproachwouldbemakingexecutorsthreadsafe.*/
if(this.serializationCheck==null){
finalResultLoaderold=this.resultLoader;
this.resultLoader=newResultLoader(old.configuration,newClosedExecutor(),old.mappedStatement,
old.parameterObject,old.targetType,old.cacheKey,old.boundSql);
}
//获取数据库查询结果并且set到结果对象返回
this.metaResultObject.setValue(property,this.resultLoader.loadResult());
}
参考地址：
https://www.cnblogs.com/qixidi/p/10251126.html
https://blog.csdn.net/mingtian625/article/details/47358003
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