loadClass（）、findClass（）、defineClass（）区别

loadClass()

• findLoadedClass(String) 调用这个方法，查看这个Class是否已经别加载

• 如果没有被加载，继续往下走，查看父类加载器，递归调用loadClass()

• 如果父类加载器是null，说明是启动类加载器，查找对应的Class

• 如果都没有找到，就调用findClass(String)

  下面是具体的源码

  protected Class<?> loadClass(String name, boolean resolve)
      throws ClassNotFoundException
  {
      synchronized (getClassLoadingLock(name)) {
          // First, check if the class has already been loaded
          Class<?> c = findLoadedClass(name);
          if (c == null) {
              long t0 = System.nanoTime();
              try {
                  if (parent != null) {
                      c = parent.loadClass(name, false);
                  } else {
                      c = findBootstrapClassOrNull(name);
                  }
              } catch (ClassNotFoundException e) {
                  // ClassNotFoundException thrown if class not found
                  // from the non-null parent class loader
              }
  
              if (c == null) {
                  // If still not found, then invoke findClass in order
                  // to find the class.
                  long t1 = System.nanoTime();
                  c = findClass(name);
  
                  // this is the defining class loader; record the stats
                  sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                  sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                  sun.misc.PerfCounter.getFindClasses().increment();
              }
          }
          if (resolve) {
              resolveClass(c);
          }
          return c;
      }
  }
  

findClass()

• 根据名称或位置加载.class字节码,然后使用defineClass
• 通常由子类去实现

protected Class<?> findClass(String name) throws ClassNotFoundException {
    throw new ClassNotFoundException(name);
}

class NetworkClassLoader extends ClassLoader {
        String host;
        int port;

         public Class findClass(String name) {
            byte[] b = loadClassData(name);
             return defineClass(name, b, 0, b.length);
         }

         private byte[] loadClassData(String name) {
             // load the class data from the connection
         }
}

definclass()

把字节码转化为Class

protected final Class<?> defineClass(String name, byte[] b, int off, int len)
    throws ClassFormatError
{
    return defineClass(name, b, off, len, null);
}