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

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[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

        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[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);

                String jarFakeUrl = 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 = 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; }