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

buser

转自：http://blog.sina.com.cn/s/blog_4d398d670100dmui.html
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:

public Class loadClass(String name, boolean resolve)             throws ClassNotFoundException {         Class clazz = 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         boolean delegateLoad = delegate || filter(name);         // (1) 先让parent加载，通常delegateLoad == false，即这一步不会执行         if (delegateLoad) {             ClassLoader loader = 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) {             ClassLoader loader = parent;             if (loader == null)                 loader = system;             clazz = loader.loadClass(name);             if (clazz != null) {                 return (clazz);             }         }         throw new ClassNotFoundException(name);     }

 
 
findClass:
 

public Class findClass(String name) 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         Class clazz = null;         clazz = findClassInternal(name);         if ((clazz == null) && hasExternalRepositories) {             synchronized (this) {                 clazz = super.findClass(name);             }         }         if (clazz == null) {             throw new ClassNotFoundException(name);         }         return (clazz);     }

 

 findClassInternal:
 

protected Class findClassInternal(String name)         throws ClassNotFoundException {         if (!validate(name))             throw new ClassNotFoundException(name);         //根据类名查找资源         String tempPath = name.replace('.', '/');         String classPath = tempPath + ".class";         ResourceEntry entry = null;         entry = findResourceInternal(name, classPath);         if (entry == null)             throw new ClassNotFoundException(name);         //如果以前已经加载成功过这个类，直接返回         Class clazz = entry.loadedClass;         if (clazz != null)             return clazz;         //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)                 throw new ClassNotFoundException(name);             // Looking up the package             String packageName = null;             int pos = name.lastIndexOf('.');             if (pos != -1)                 packageName = name.substring(0, pos);             Package pkg = 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,                             new CodeSource(entry.codeBase, entry.certificates));                 } catch (UnsupportedClassVersionError ucve) {                     throw new 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;             }         }         return clazz;     }

 
 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, String path) {
 
//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 + 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, path);
             entry = (ResourceEntry)AccessController.doPrivileged(dp);
        } else {
             entry = findResourceInternal(files, 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
        ResourceAttributes attributes = (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.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, false);
                String jarFakeUrl = getURI(jarRealFiles).toString();
                jarFakeUrl = "jar:" + jarFakeUrl + "!/" + path;
                entry.source = new URL(jarFakeUrl);
                entry.lastModified = jarRealFiles.lastModified();
             } catch (MalformedURLException e) {
                  return null;
              }
            contentLength = (int) jarEntry.getSize();
            try {
                  entry.manifest = jarFiles.getManifest();
                  binaryStream = jarFiles.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 = new byte[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; }