jvm原理（8）不同的类加载器与加载与加载动作分析

这段程序打印出系统类加载器到最上层的加载器的结构关系：

public class MyTest13 {
<!-- more -->
    public static void main(String[] args) {
        ClassLoader classLoader = ClassLoader.getSystemClassLoader();
        System.out.println(classLoader);

        while(null != classLoader){
            classLoader = classLoader.getParent();
            System.out.println(classLoader);
        }

    }
}

运行结果：

sun.misc.Launcher$AppClassLoader@18b4aac2 【系统类加载器】
sun.misc.Launcher$ExtClassLoader@1540e19d  【扩展类加载器】
null                                       【根类加载器】

可以看到最后输出的是null。
这行代码： ClassLoader classLoader = ClassLoader.getSystemClassLoader();得到系统类加载器，看一下doc说明：

/**
     * Returns the system class loader for delegation.  This is the default
     * delegation parent for new <tt>ClassLoader</tt> instances, and is
     * typically the class loader used to start the application.
     *返回一个针对委托的系统类加载器，并且他是默认新建类加载器实例的委托双亲（即自定义类加载器的父级，见下图），它是一个典型的启动应用的类加载器。
     * <p> This method is first invoked early in the runtime's startup
     * sequence, at which point it creates the system class loader and sets it
     * as the context class loader of the invoking <tt>Thread</tt>.
     *此方法在运行期的早期就会被调用，在这个时间点创建系统类的加载器，并且设定其为调用线程的上下文的一个类加载器。
     * <p> The default system class loader is an implementation-dependent
     * instance of this class.
     *默认的系统类加载器与这个类实现相关的实例
     * <p> If the system property "<tt>java.system.class.loader</tt>" is defined
     * when this method is first invoked then the value of that property is
     * taken to be the name of a class that will be returned as the system
     * class loader.  The class is loaded using the default system class loader
     * and must define a public constructor that takes a single parameter of
     * type <tt>ClassLoader</tt> which is used as the delegation parent.  An
     * instance is then created using this constructor with the default system
     * class loader as the parameter.  The resulting class loader is defined
     * to be the system class loader.
     * 如果设定了java.system.class.loader那么这个方法返回的就是java.system.class.loader设定的类加载器。这个类被系统类加载器加载，并且
     * 定义一个公共的构造方法，接受一个ClassLoader参数用作为委托的双亲，用默认系统类类加载器作为构造器的参数，就会创造一个实例 ，所得到的就是系统类加载器
     */
     //返回系统类加载器
         @CallerSensitive
    public static ClassLoader getSystemClassLoader() {
        initSystemClassLoader();
        if (scl == null) {
            return null;
        }
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkClassLoaderPermission(scl, Reflection.getCallerClass());
        }
        return scl;
    }

getParent方法：

/**
     * Returns the parent class loader for delegation. Some implementations may
     * use <tt>null</tt> to represent the bootstrap class loader. This method
     * will return <tt>null</tt> in such implementations if this class loader's
     * parent is the bootstrap class loader.
     * 返回父加载器用于委托，有些实现返回null用来表示根类加载器，如果一个类的父加载器是根加载器，那么这个方法将会返回null
     */
      public final ClassLoader getParent() {
        if (parent == null)
            return null;
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            // Check access to the parent class loader
            // If the caller's class loader is same as this class loader,
            // permission check is performed.
            checkClassLoaderPermission(parent, Reflection.getCallerClass());
        }
        return parent;
    }

parent 变量是ClassLoader的成员变量：

// The parent class loader for delegation
// Note: VM hardcoded the offset of this field, thus all new fields
// must be added *after* it.
用于委托的双亲加载器，JVM将这个变量的偏移量进行了硬编码，，这样新的变量就要加载这个变量的后边
private final ClassLoader parent;

下一个例子：

public class MyTest14 {
    public static void main(String[] args) throws IOException {
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        String resourceName = "com/twodragonlake/jvm/classloader/MyTest13.class";
        /*
        Finds all the resources with the given name. A resource is some data
        (images, audio, text, etc) that can be accessed by class code in a way
        that is independent of the location of the code.
        返回给定名字所有的资源，资源可以是（图片，音频，文本，等）可以被class字节码以一种与字节码位置无关的方式去访问，
        classLoader.getResources(resourceName){....}
		*/
        Enumeration<URL> urls =  classLoader.getResources(resourceName);
        while(urls.hasMoreElements()){
            System.out.println(urls.nextElement());
        }

        System.out.println("----------------");

        Class<?> clazz = String.class;
        System.out.println(clazz.getClassLoader());//自定义的类有系统类加载器加载
        System.out.println("----------------");
        clazz = MyTest14.class;
        System.out.println(clazz.getClassLoader());  //由根类加载器加载 因为系统类加载器的加载目录包含rt目录

    }
}

打印：

file:/E:/Study/intelIde/jvm_lecture/out/production/classes/com/twodragonlake/jvm/classloader/MyTest13.class
----------------
null
----------------
sun.misc.Launcher$AppClassLoader@18b4aac2

获取ClassLoader的方式：
获得当前类的ClassLoader：
class.getClassLoader();
获取当前线程上下文的ClassLoader：
Thread.currentThread().getContextClassLoader();
获得系统的ClassLoader：
ClassLoader.getSystemClassLoader()
获得调用者的ClassLoader:
DriverManager.getCallerClassLoader();