Tomcat的WebappClassLoader(web应用类加载器)详解(一)

Tomcat负责Web应用的类加载的是org.apache.catalina.loader.WebappClassLoader，它有几个比较重要的方法：findClass(),loadClass(),findClassInternal(),findResourceInternal().

WebappClassLoader类加载器被用来加载一个类的时候，loadClass()会被调用，loadClass()则调用findClass()。后两个方法是WebappClassLoader的私有方法，findClass()调用findClassInternal()来创建class对象，而findClassInternal()则需要findResourceInternal()来查找.class文件。

通常自己实现类记载器只要实现findclass即可，这里为了实现特殊目的而override了loadClass().

下面是精简过的代码（去除了几乎全部关于log、异常和安全控制的代码）：

loadClass:

publicClass loadClass(Stringname, boolean resolve) throwsClassNotFoundException{ Classclazz = null; //(0) 先从自己的缓存中查找，有则返回，无则继续 clazz= findLoadedClass0(name); if(clazz !=null) { if(resolve) resolveClass(clazz); return(clazz); } //(0.1) 再从parent的缓存中查找 clazz= findLoadedClass(name); if(clazz !=null) { if(resolve) resolveClass(clazz); return(clazz); } //(0.2) 缓存中没有，则首先使用system类加载器来加载 clazz =system.loadClass(name); if (clazz !=null) { if(resolve) resolveClass(clazz); return(clazz); } //判断是否需要先让parent代理 booleandelegateLoad = delegate ||filter(name); //(1) 先让parent加载，通常delegateLoad ==false，即这一步不会执行 if(delegateLoad) { ClassLoaderloader = parent; if(loader ==null) loader= system; clazz= loader.loadClass(name); if(clazz !=null) { if(resolve) resolveClass(clazz); return(clazz); } } //(2) delegateLoad == false 或者 parent加载失败，调用自身的加载机制 clazz= findClass(name); if(clazz !=null) { if(resolve) resolveClass(clazz); return(clazz); } //(3) 自己加载失败，则请求parent代理加载 if(!delegateLoad) { ClassLoaderloader = parent; if(loader ==null) loader= system; clazz= loader.loadClass(name); if(clazz !=null) { return(clazz); } } thrownew ClassNotFoundException(name); }

findClass:

public Class findClass(Stringname) throws ClassNotFoundException { //先试图自己加载类，找不到则请求parent来加载 //注意这点和java默认的双亲委托模式不同 Classclazz = null; clazz= findClassInternal(name); if((clazz ==null) &&hasExternalRepositories){ synchronized(this){ clazz= super.findClass(name); } } if(clazz ==null) { thrownew ClassNotFoundException(name); } return(clazz); }

findClassInternal:

protected Class findClassInternal(Stringname) throwsClassNotFoundException{ if(!validate(name)) thrownew ClassNotFoundException(name); //根据类名查找资源 StringtempPath = name.replace('.','/'); StringclassPath = tempPath+ ".class"; ResourceEntryentry = null; entry=findResourceInternal(name,classPath); if(entry ==null) thrownew ClassNotFoundException(name); //如果以前已经加载成功过这个类，直接返回 Classclazz = entry.loadedClass; if(clazz !=null) returnclazz; //以下根据找到的资源(.class文件)进行:1、定义package;2、对package安全检查;3、定义class，即创建class对象 synchronized(this){ if(entry.binaryContent ==null && entry.loadedClass ==null) thrownew ClassNotFoundException(name); //Looking up the package StringpackageName = null; intpos = name.lastIndexOf('.'); if(pos !=-1) packageName= name.substring(0,pos); Packagepkg = null; if(packageName !=null) { pkg= getPackage(packageName); //Define the package (if null) if(pkg ==null) { //定义package的操作，此处省略，具体参看源码 pkg= getPackage(packageName); } } if(securityManager !=null) { //安全检查操作，此处省略，具体参看源码 } //创建class对象并返回 if(entry.loadedClass ==null) { try{ clazz= defineClass(name,entry.binaryContent, 0, entry.binaryContent.length, newCodeSource(entry.codeBase, entry.certificates)); }catch (UnsupportedClassVersionErrorucve) { thrownew UnsupportedClassVersionError( ucve.getLocalizedMessage()+ " " + sm.getString("webappClassLoader.wrongVersion", name)); } entry.loadedClass= clazz; entry.binaryContent= null; entry.source= null; entry.codeBase= null; entry.manifest= null; entry.certificates= null; }else { clazz= entry.loadedClass; } } returnclazz; }

findResouceInternal():

//要先加载相关实体资源(.jar) 再加载查找的资源本身

protected ResourceEntry findResourceInternal(String name, Stringpath) {

//先根据类名从缓存中查找对应资源 ，有则直接返回

ResourceEntry entry = (ResourceEntry)resourceEntries.get(name);

if (entry != null) return entry; int contentLength = -1;

//资源二进制数据长度

InputStream binaryStream = null;

//资源二进制输入流

int jarFilesLength = jarFiles.length;

//classpath中的jar包个数

int repositoriesLength = repositories.length;

//仓库数(classpath每一段称为repository仓库)

int i; Resource resource = null;

//加载的资源实体

boolean fileNeedConvert = false;

//对每个仓库迭代，直接找到相应的entry，如果查找的资源是一个独立的文件，在这个代码块可以查找到相应资源，

//如果是包含在jar包中的类，这段代码并不能找出其对应的资源

for (i = 0; (entry == null) &&(i < repositoriesLength); i++) {

try {

String fullPath = repositories[i] + path;

//仓库路径 加资源路径得到全路径

Object lookupResult = resources.lookup(fullPath);

//从资源库中查找资源

if (lookupResult instanceof Resource) {

resource = (Resource) lookupResult;

}

//到这里没有抛出异常，说明资源已经找到，现在构造entry对象

if (securityManager != null) {

PrivilegedAction dp = new PrivilegedFindResource(files[i],path);

entry = (ResourceEntry)AccessController.doPrivileged(dp);

} else {

entry = findResourceInternal(files[i], path);

//这个方式只是构造entry并给其codebase和source赋值

}

//获取资源长度和最后修改时间

ResourceAttributesattributes = (ResourceAttributes)resources.getAttributes(fullPath);

contentLength = (int) attributes.getContentLength();

entry.lastModified= attributes.getLastModified();

//资源找到，将二进制输入流赋给binaryStream

if (resource != null) {

try {

binaryStream = resource.streamContent();

} catch (IOException e) {

return null;

}

//将资源的最后修改时间加到列表中去，代码略去，参加源码

}

} catch (NamingException e) { }

}

if ((entry == null) &&(notFoundResources.containsKey(name)))

return null;

//开始从jar包中查找

JarEntry jarEntry =null;

synchronized (jarFiles) {

if (!openJARs()) { return null; }

for(i = 0; (entry == null) && (i< jarFilesLength); i++) {

jarEntry = jarFiles[i].getJarEntry(path);

//根据路径从jar包中查找资源

//如果jar包中找到资源，则定义entry并将二进制流等数据赋给entry，同时将jar包解压到workdir.

if(jarEntry != null) {

entry = new ResourceEntry();

try {

entry.codeBase = getURL(jarRealFiles[i], false);

StringjarFakeUrl = getURI(jarRealFiles[i]).toString();

jarFakeUrl = "jar:" + jarFakeUrl + "!/" + path;

entry.source= new URL(jarFakeUrl);

entry.lastModified= jarRealFiles[i].lastModified();

} catch (MalformedURLException e) {

return null;

}

contentLength = (int) jarEntry.getSize();

try {

entry.manifest = jarFiles[i].getManifest();

binaryStream= jarFiles[i].getInputStream(jarEntry);

}catch (IOException e) { return null; }

if (antiJARLocking &&!(path.endsWith(".class"))) {

//解压jar包代码，参见源码 }

}

}

if (entry == null) { synchronized (notFoundResources) {notFoundResources.put(name, name); } return null; } //从二进制流将资源内容读出if (binaryStream != null) { byte[] binaryContent = newbyte[contentLength]; //读二进制流的代码，这里省去，参见源码 entry.binaryContent
=binaryContent; } } // 将资源加到缓存中 synchronized (resourceEntries) {ResourceEntry entry2 = (ResourceEntry) resourceEntries.get(name);if (entry2 == null) { resourceEntries.put(name, entry); } else {entry = entry2; } } return entry; }