1.系統(tǒng)默認類加載器

BootStrap加載器：主要加載rt.jar，它是Java基礎(chǔ)類庫，里面有String等類(String.class.getClassLoader() = null)
Extension加載器：主要加載jre/lib/ext目錄下jar，其中包含Java的一些加密方法等
Application(System)加載器：加載classpath下jar      

2.類加載器三種機制

委托：當加載一個Class時，當前加載器會先委托父加載器加載 如果父加載器沒能加載 則自己加載
可見：父加載器加載的Class，對子加載器都是可見的，而子類加載的Class父加載器不知道
單一：一個Class只能被加載一次

3.看看一下的例子

3.1

@Test
public void testBootstrapClassLoader() {
    ClassLoader classLoader = String.class.getClassLoader();
    try {
        Class<?> loadClass = classLoader.loadClass("com.boomsecret.domain.Person");
        Object obj = loadClass.newInstance();
        Method method = loadClass.getMethod("setUsername", String.class);
        method.invoke(obj, "Bootstrap ClassLoader");
        Method getUsernameMethod = loadClass.getMethod("getUsername");
        Object invoke = getUsernameMethod.invoke(obj);
        System.out.println(invoke);
        System.out.println(obj);
    } catch (Exception e) {
        System.out.println(e);
    }
}

3.2

@Test
public void testExtensionClassLoader() {
    ClassLoader classLoader = AccessBridge.class.getClassLoader();
    try {
        Class<?> loadClass = classLoader.loadClass("com.boomsecret.domain.Person");
        Object obj = loadClass.newInstance();
        Method method = loadClass.getMethod("setUsername", String.class);
        method.invoke(obj, "Extension ClassLoader");
        Method getUsernameMethod = loadClass.getMethod("getUsername");
        Object invoke = getUsernameMethod.invoke(obj);
        System.out.println(invoke);
        System.out.println(obj);
    } catch (Exception e) {
        System.out.println(e);
    }
}

ClassNotFoundException：(AccessBridge是jre/lib/ext中類)所以取到的是Extension加載器，自然是找不到com.boomsecret.domain.Person的

3.3

@Test
public void testApplicationClassLoader() {
    ClassLoader classLoader = getClass().getClassLoader();
    try {
        Class<?> loadClass = classLoader.loadClass("com.boomsecret.domain.Person");
        Object obj = loadClass.newInstance();
        Method method = loadClass.getMethod("setUsername", String.class);
        method.invoke(obj, "Application ClassLoader");
        Method getUsernameMethod = loadClass.getMethod("getUsername");
        Object invoke = getUsernameMethod.invoke(obj);
        System.out.println(invoke);
        System.out.println(obj);
    } catch (Exception e) {
        System.out.println(e);
    }
}

正常：Application加載器加載classpath中類

3.4 顯示加載Class Api1

@Test
public void testLoadClass() {
    try {
        Class<?> loadClass = Class.forName("com.boomsecret.domain.Person");
        Object obj = loadClass.newInstance();
        Method method = loadClass.getMethod("setUsername", String.class);
        method.invoke(obj, "Application ClassLoader");
        Method getUsernameMethod = loadClass.getMethod("getUsername");
        Object invoke = getUsernameMethod.invoke(obj);
        System.out.println(invoke);
        System.out.println(obj);
    } catch (Exception e) {
        System.out.println(e);
    }
}

3.5 顯示加載Class APi2

@Test
public void testLoadClass2() {
    try {
        Class<?> loadClass = Class.forName("com.boomsecret.domain.Person",
                true, getClass().getClassLoader());
        Object obj = loadClass.newInstance();
        Method method = loadClass.getMethod("setUsername", String.class);
        method.invoke(obj, "Application ClassLoader");
        Method getUsernameMethod = loadClass.getMethod("getUsername");
        Object invoke = getUsernameMethod.invoke(obj);
        System.out.println(invoke);
        System.out.println(obj);
    } catch (Exception e) {
        System.out.println(e);
    }
}