Java虚拟机学习 - 类加载器(ClassLoader)

类加载器

类与类加载器

类加载器分类：

主要分为Bootstrap ClassLoader、Extension ClassLoader、Application ClassLoader和User Defined ClassLoader。

启动类加载器(Bootstrap ClassLoader)：

扩展类加载器(Extension ClassLoader)：

应用程序类加载器(Application ClassLoader)：

自定义类加载器(User Defined ClassLoader)：

双亲委托模型：

public abstract class ClassLoader {

    private static native void registerNatives();
    static {
        registerNatives();
    }

    // The parent class loader for delegation
    private ClassLoader parent;

    // Hashtable that maps packages to certs
    private Hashtable package2certs = new Hashtable(11);
}

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类加载器SPI：

    protected synchronized Class<?> loadClass(String name, boolean resolve)
	throws ClassNotFoundException
    {
	// First, check if the class has already been loaded
	Class c = findLoadedClass(name);
	if (c == null) {
	    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.
	        c = findClass(name);
	    }
	}
	if (resolve) {
	    resolveClass(c);
	}
	return c;
    }

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    protected final Class<?> findLoadedClass(String name) {
	if (!checkName(name))
	    return null;
	return findLoadedClass0(name);
    }

    private native final Class findLoadedClass0(String name);

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

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    protected final Class<?> findSystemClass(String name)
	throws ClassNotFoundException
    {
	ClassLoader system = getSystemClassLoader();
	if (system == null) {
	    if (!checkName(name))
		throw new ClassNotFoundException(name);
            Class cls = findBootstrapClass(name);
            if (cls == null) {
                throw new ClassNotFoundException(name);
            } 
	    return cls;
	}
	return system.loadClass(name);
    }

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

    // Private method w/ an extra argument for skipping class verification
    private final Class<?> defineClassCond(String name,
                                           byte[] b, int off, int len,
                                           ProtectionDomain protectionDomain,
                                           boolean verify)
        throws ClassFormatError
    {
	protectionDomain = preDefineClass(name, protectionDomain);

	Class c = null;
        String source = defineClassSourceLocation(protectionDomain);

	try {
	    c = defineClass1(name, b, off, len, protectionDomain, source,
                             verify);
	} catch (ClassFormatError cfe) {
	    c = defineTransformedClass(name, b, off, len, protectionDomain, cfe,
                                       source, verify);
	}

	postDefineClass(c, protectionDomain);
	return c;
    }

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常见异常：

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