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

最新推荐文章于 2025-03-02 10:18:20 发布

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原文链接：https://www.jianshu.com/p/b75a89014e27

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本文深入探讨了Java类加载过程，详细解释了loadClass(), findLoadedClass(), findClass()及defineClass()等核心方法的工作原理。通过源码分析，揭示了类加载器如何递归调用父类加载器，以及在找不到类时的处理策略。

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);
}