Android的ClassLoader知多少

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 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);
            }
        }
        return c;
}
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能够看出loadClass的基本流程以下：

首先判断是否已经加载过class，若是加载过，就直接返回对应的Class<?>对象；若是没有加载过，就查找它的父类是否加载过，若是父类加载过，就返回；若是父类未加载过，就本身去加载。

Android应用中ClassLoader对象

public class MainActivity extends AppCompatActivity {

    private static final String TAG = MainActivity.class.getSimpleName();

    @Override
    protected 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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咱们建立一个Android项目，在Activity的onCreate方法中打印ClassLoader的值，如上。

打印结果以下：

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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由打印日志能够看出，此App包含了两种ClassLoader，一个是PathClassLoader，一个是BootClassLoader。

BootClassLoader是系统启动时建立的,用于加载Android FrameWork层的class；PathClassLoader是在App启动时建立的，用于加载该App中的class文件。

ClassLoader的做用

1，共享做用：类加载过，后面用到时就不用再加载。

2，隔离做用：不一样继承路线的类加载器，加载的类必定是不一样的。

DexClassLoader与PathClassLoader异同点

DexClassLoader和PathClassLoader都是继承BaseDexClassLoader。

DexClassLoader：可加载jar/dex/apk，也能够加载SD卡中未安装的apk。

PathClassLoader：只可加载系统中已经安装的apk。