loadClass基本流程

/*** Loads the class with the specified <a href="#name">binary name</a>.  The*/
protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException
{// First, check if the class has already been loadedClass<?> 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);}}return c;
}
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可以看出loadClass的基本流程如下：

首先判断是否已经加载过class，如果加载过，就直接返回对应的Class<?>对象；如果没有加载过，就查找它的父类是否加载过，如果父类加载过，就返回；如果父类未加载过，就自己去加载。

Android应用中ClassLoader对象

public class MainActivity extends AppCompatActivity {private static final String TAG = MainActivity.class.getSimpleName();@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);printClassLoaders();setContentView(R.layout.activity_main);}private void printClassLoaders() {ClassLoader classLoader = getClassLoader();if (classLoader != null) {Log.e(TAG, "Current ClassLoader is: " + classLoader.toString());while (classLoader.getParent() != null) {classLoader = classLoader.getParent();Log.e(TAG, "ClassLoader's parent is: " + classLoader.toString());}}}
}
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我们创建一个Android项目，在Activity的onCreate方法中打印ClassLoader的值，如上。

打印结果如下：

2019-04-30 18:52:53.219 16456-16456/com.my.classloaderdemo E/MainActivity: Current ClassLoader is: dalvik.system.PathClassLoader[DexPathList[[zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/base.apk", zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/split_lib_dependencies_apk.apk", zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/split_lib_resources_apk.apk", zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/split_lib_slice_0_apk.apk", zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/split_lib_slice_1_apk.apk", zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/split_lib_slice_2_apk.apk", zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/split_lib_slice_3_apk.apk", zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/split_lib_slice_4_apk.apk", zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/split_lib_slice_5_apk.apk", zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/split_lib_slice_6_apk.apk", zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/split_lib_slice_7_apk.apk", zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/split_lib_slice_8_apk.apk", zip file "/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/split_lib_slice_9_apk.apk"],nativeLibraryDirectories=[/data/app/com.my.classloaderdemo-faSliZa20JPLlSUl0kPawg==/lib/x86, /system/lib, /vendor/lib]  ]
]2019-04-30 18:52:53.219 16456-16456/com.my.classloaderdemo E/MainActivity: ClassLoader's parent is: java.lang.BootClassLoader@c8bb22f
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由打印日志可以看出，此App包含了两种ClassLoader，一个是PathClassLoader，一个是BootClassLoader。

BootClassLoader是系统启动时创建的,用于加载Android FrameWork层的class；PathClassLoader是在App启动时创建的，用于加载该App中的class文件。

ClassLoader的作用

1，共享作用：类加载过，后面用到时就不用再加载。

2，隔离作用：不同继承路线的类加载器，加载的类一定是不同的。

DexClassLoader与PathClassLoader异同点

DexClassLoader和PathClassLoader都是继承BaseDexClassLoader。

DexClassLoader：可加载jar/dex/apk，也可以加载SD卡中未安装的apk。

PathClassLoader：只可加载系统中已经安装的apk。