public abstract class ClassLoader {
  //  每個(gè)類加載器都有一個(gè)父加載器
  private final ClassLoader parent;
  public Class<?> loadClass(String name) throws ClassNotFoundException {
        return loadClass(name, false);
    }
     protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
           // 如果沒(méi)有加載過(guò)
            if (c == null) {
                if (parent != null) {
                  //  先委托給父加載器去加載，注意這是個(gè)遞歸調(diào)用
                 c = parent.loadClass(name, false);
                } else {
                 // 如果父加載器為空，查找 Bootstrap 加載器是不是加載過(guò)了
                   c = findBootstrapClassOrNull(name);
                }
              
            // 如果父加載器沒(méi)加載成功，調(diào)用自己的 findClass 去加載
                if (c == null) {        
                    c = findClass(name);
                }
            } 
        
            return c;
        }
        
    }
    //ClassLoader 中findClass方式需要被子類覆蓋，下面這段代碼就是對(duì)應(yīng)代碼
      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<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
        synchronized (getClassLoadingLock(name)) {
            if (log.isDebugEnabled())
                log.debug("loadClass(" + name + ", " + resolve + ")");
            Class<?> clazz = null;
            // Log access to stopped class loader
            checkStateForClassLoading(name);    
            //從當(dāng)前ClassLoader的本地緩存中加載類，如果找到則返回
            clazz = findLoadedClass0(name);
            if (clazz != null) {
                if (log.isDebugEnabled())
                    log.debug("  Returning class from cache");
                if (resolve)
                    resolveClass(clazz);
                return clazz;
            }
            // 本地緩存沒(méi)有的情況下，調(diào)用ClassLoader的findLoadedClass方法查看jvm是否已經(jīng)加載過(guò)此類，如果已經(jīng)加載則直接返回。
            clazz = findLoadedClass(name);
            if (clazz != null) {
                if (log.isDebugEnabled())
                    log.debug("  Returning class from cache");
                if (resolve)
                    resolveClass(clazz);
                return clazz;
            }
            String resourceName = binaryNameToPath(name, false);
            //此時(shí)的javaseClassLoader是擴(kuò)展類加載器  是把擴(kuò)展類加載器賦值給了javaseClassLoader
            ClassLoader javaseLoader = getJavaseClassLoader();
            boolean tryLoadingFromJavaseLoader;
            try {
              .....
            //如果可以用getResource得到
            //如果能用擴(kuò)展類加載器的getResource得到就證明可以被擴(kuò)展類加載器加載到接下來(lái)安排擴(kuò)展類加載器加載
            if (tryLoadingFromJavaseLoader) {
                try {
                    //使用擴(kuò)展類加載器進(jìn)行加載
                    clazz = javaseLoader.loadClass(name);
                    if (clazz != null) {
                        if (resolve)
                            resolveClass(clazz);
                        return clazz;
                    }
                } catch (ClassNotFoundException e) {
                    // Ignore
                }
            }
            // (0.5) Permission to access this class when using a SecurityManager
            if (securityManager != null) {
                int i = name.lastIndexOf('.');
                if (i >= 0) {
                    try {
                        securityManager.checkPackageAccess(name.substring(0,i));
                    } catch (SecurityException se) {
                        String error = "Security Violation, attempt to use " +
                            "Restricted Class: " + name;
                        log.info(error, se);
                        throw new ClassNotFoundException(error, se);
                    }
                }
            }
            boolean delegateLoad = delegate || filter(name, true);
            // (1) Delegate to our parent if requested
            //如果是true就是用父類加載器進(jìn)行加載
            if (delegateLoad) {
                if (log.isDebugEnabled())
                    log.debug("  Delegating to parent classloader1 " + parent);
                try {
                    clazz = Class.forName(name, false, parent);
                    if (clazz != null) {
                        if (log.isDebugEnabled())
                            log.debug("  Loading class from parent");
                        if (resolve)
                            resolveClass(clazz);
                        return clazz;
                    }
                } catch (ClassNotFoundException e) {
                    // Ignore
                }
            }
            // (2) Search local repositories
            if (log.isDebugEnabled())
                log.debug("  Searching local repositories");
            try {
                // 本地進(jìn)行加載
                clazz = findClass(name);
                if (clazz != null) {
                    if (log.isDebugEnabled())
                        log.debug("  Loading class from local repository");
                    if (resolve)
                        resolveClass(clazz);
                    return clazz;
                }
            } catch (ClassNotFoundException e) {
                // Ignore
            }
            // (3) Delegate to parent unconditionally
            //到這里還是沒(méi)有加載上再次嘗試使用父類加載器進(jìn)行加載
            if (!delegateLoad) {
                    if (log.isDebugEnabled())
                    log.debug("  Delegating to parent classloader at end: " + parent);
                try {
                    clazz = Class.forName(name, false, parent);
                    if (clazz != null) {
                        if (log.isDebugEnabled())
                            log.debug("  Loading class from parent");
                        if (resolve)
                            resolveClass(clazz);
                        return clazz;
                    }
                } catch (ClassNotFoundException e) {
                    // Ignore
                }
            }
        }
        throw new ClassNotFoundException(name);
    }