淺談Tomcat如何打破雙親委托機(jī)制

public abstract class ClassLoader {

    // 每個(gè)類加載器都有個(gè)父加載器
    private final ClassLoader parent;
    
    public Class<?> loadClass(String name) {
  
        // 查找該類是否被加載過(guò)
        Class<?> c = findLoadedClass(name);
        
        // 若未被加載過(guò)
        if( c == null ){
          // 【遞歸】委托給父加載器加載
          if (parent != null) {
              c = parent.loadClass(name);
          } else {
              // 若父加載器為空，查找Bootstrap加載器是否加載過(guò)了
              c = findBootstrapClassOrNull(name);
          }
        }
        // 若父加載器未加載成功，調(diào)用自己的findClass去加載
        if (c == null) {
            c = findClass(name);
        }
        
        return c；
    }
    
    protected Class<?> findClass(String name){
       // 1. 根據(jù)傳入的類名name，到在特定目錄下去尋找類文件，把.class文件讀入內(nèi)存
          ...
          
       // 2. 調(diào)用defineClass將字節(jié)數(shù)組轉(zhuǎn)成Class對(duì)象
       return defineClass(buf, off, len)；
    }
    
    // 將字節(jié)碼數(shù)組解析成一個(gè)Class對(duì)象，用native方法實(shí)現(xiàn)
    protected final Class<?> defineClass(byte[] b, int off, int len){
       ...
    }
}

public Class<?> findClass(String name) throws ClassNotFoundException {
    ...
    
    Class<?> clazz = null;
    try {
            //1. 先在Web應(yīng)用目錄下查找類 
            clazz = findClassInternal(name);
    }  catch (RuntimeException e) {
           throw e;
       }
    
    if (clazz == null) {
    try {
            //2. 如果在本地目錄沒(méi)有找到，交給父加載器去查找
            clazz = super.findClass(name);
    }  catch (RuntimeException e) {
           throw e;
       }
    
    //3. 如果父類也沒(méi)找到，拋出ClassNotFoundException
    if (clazz == null) {
        throw new ClassNotFoundException(name);
     }

    return clazz;
}

public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {

    synchronized (getClassLoadingLock(name)) {
 
        Class<?> clazz = null;

        //1. 先在本地cache查找該類是否已經(jīng)加載過(guò)
        clazz = findLoadedClass0(name);
        if (clazz != null) {
            if (resolve)
                resolveClass(clazz);
            return clazz;
        }

        //2. 從系統(tǒng)類加載器的cache中查找是否加載過(guò)
        clazz = findLoadedClass(name);
        if (clazz != null) {
            if (resolve)
                resolveClass(clazz);
            return clazz;
        }

        // 3. 嘗試用ExtClassLoader類加載器類加載，為什么？
        ClassLoader javaseLoader = getJavaseClassLoader();
        try {
            clazz = javaseLoader.loadClass(name);
            if (clazz != null) {
                if (resolve)
                    resolveClass(clazz);
                return clazz;
            }
        } catch (ClassNotFoundException e) {
            // Ignore
        }

        // 4. 嘗試在本地目錄搜索class并加載
        try {
            clazz = findClass(name);
            if (clazz != null) {
                if (resolve)
                    resolveClass(clazz);
                return clazz;
            }
        } catch (ClassNotFoundException e) {
            // Ignore
        }

        // 5. 嘗試用系統(tǒng)類加載器(也就是AppClassLoader)來(lái)加載
            try {
                clazz = Class.forName(name, false, parent);
                if (clazz != null) {
                    if (resolve)
                        resolveClass(clazz);
                    return clazz;
                }
            } catch (ClassNotFoundException e) {
                // Ignore
            }
       }
    
    //6. 上述過(guò)程都加載失敗，拋出異常
    throw new ClassNotFoundException(name);
}