Android 動態加載之DexClassLoader

Android提供動態加載機制，允許從SD卡中加載dex格式的文件，其中，DexClassLoader類起了關鍵作用。

首先看下Android Developer關於DexClassLoader的介紹，

A class loader that loads classes from .jar and .apk files containing a classes.dex entry. This can be used to execute code not installed as part of an application.
This class loader requires an application-private, writable directory to cache optimized classes. Use Context.getCodeCacheDir() to create such a directory:

File dexOutputDir = context.getCodeCacheDir();

Do not cache optimized classes on external storage. External storage does not provide access controls necessary to protect your application from code injection attacks.

簡單翻譯一下，DexClassLoader是一個加載.jar或.apk文件的類加載器，其中該文件是dex格式。其可以用來執行程序之外的其他代碼。該加載器需要一個程序私有的文件，Google推薦使用context.getCodeChaceDir()目錄來使用。

我們來看下其構造函數，

public DexClassLoader (String dexPath, String optimizedDirectory, String libraryPath, ClassLoader parent)

Added in API level 3

Creates a DexClassLoader that finds interpreted and native code. Interpreted classes are found in a set of DEX files contained in Jar or APK files.

The path lists are separated using the character specified by the path.separator system property, which defaults to :.

Parameters

dexPath	the list of jar/apk files containing classes and resources, delimited by `File.pathSeparator`, which defaults to `":"` on Android
optimizedDirectory	directory where optimized dex files should be written; must not be `null`
libraryPath	the list of directories containing native libraries, delimited by `File.pathSeparator`; may be `null`
parent	the parent class loader

其中，dexPath指定了要加載的文件目錄；optimizedDirectory是優化文件存放的路徑，也就是上面提到的應該是程序私有的路徑（其他程序無法訪問的），libraryPath包含的native庫路徑，parent，指定父加載器。

該類除了構造器之外，沒有其他方法，我們看下其繼承關係。

java.lang.Object
↳	java.lang.ClassLoader
	↳	dalvik.system.BaseDexClassLoader
		↳	dalvik.system.DexClassLoader

看到，其繼承了BaseDexClassLoader類，BaseDexClassLoader繼承了ClassLoaer類，我們找到其源碼，進行分析，

1. DexClassLoader類只有一個構造函數，將4個構造參數傳給BaseDexClassLoader，接下來分析BaseDexClassLoader，

2. BaseDexClassLoader有兩個成員變量，String originalPath和DexPathList pathList；看其註釋，originalPath指的是其文件路徑，pathlist是路徑元素列表（原文是，structured lists of path elements，下文會分析），BaseDexClassLoader重載了ClassLoader父類的一些方法，我們注重分析findclass，其代碼如下，

@Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        Class clazz = pathList.findClass(name);
 
        if (clazz == null) {
            throw new ClassNotFoundException(name);
        }
 
        return clazz;
    }

然而其並沒有重載父類的loadClass方法（加載類時最主要用到的方法），我們接着分析ClassLoader類

3. ClassLoader

其包含了很多方法，我們注重分析loadClass方法，

protected Class<?> loadClass(String className, boolean resolve) throws ClassNotFoundException {
        Class<?> clazz = findLoadedClass(className);
 
        if (clazz == null) {
            try {
                clazz = parent.loadClass(className, false);
            } catch (ClassNotFoundException e) {
                // Don't want to see this.
            }
 
            if (clazz == null) {
                clazz = findClass(className);
            }
        }
 
        return clazz;
    }

我們可以看到，當loadClass時，會先判斷是否已加載過，若加載過，則不需要重新加載，若沒有加載過，則調用parent.loadClass方法，若其返回爲null（說明parent未加載成功），則調用findClass自己加載，上面的機制稱爲雙親委派機制。+

好，接下來我們測試一下DexClassLoader能否成功加載外部代碼。

1. 生成外部加載代碼

我們新建一個Android工程，其目錄如下，

其中，ILib定義了一個接口，關於定義接口的好處，下面會提到。

public interface ILib {
	public void sayHello();
}

Lib實現了ILIb接口，在sayHello中輸出一句話

public class Lib implements ILib {
 
	@Override
	public void sayHello() {
		// TODO Auto-generated method stub
		System.out.println("Hello from Thrid Dex Lib");
	}
 
}

然後，讓我們導出Lib類，選中Lib.java，右鍵export，選擇java-jar file導出，命名爲lib.jar，注意導出時，不要選擇接口文件（ILib），否則會出錯。

上面提到DexClassLoader加載的是Dex格式的文件，但是Eclipse導出的是jar格式，所以，我們得用Android的dx命令來將lib.jar編譯爲dex格式。dx命令位於Android sdk/build-tools/版本號/目錄，爲了方便可以將其路徑加入到系統變量中，然後在cmd中切換到lib.jar所在目錄，執行一下命令

dx --dex --output=mylib.jar lib.jar

得到mylib.jar文件

2. 動態加載

得到test.jar文件後，我們將其放入到工程的aseet目錄，由於DexClassLoader是從SD卡中加載代碼的，所以我們在程序運行的時候，將test.jar從asset目錄copy到SD卡中，注意copy到的路徑必須是該程序私有的。

private void copy2SDCard(File dexInternalStoragePath) {
		BufferedInputStream bis;
		try {
			bis = new BufferedInputStream(getAssets().open(LIB_JAR));
			OutputStream dexWriter = new BufferedOutputStream(
					new FileOutputStream(dexInternalStoragePath));
			byte[] buf = new byte[BUF_SIZE];
			int len;
			while ((len = bis.read(buf, 0, BUF_SIZE)) > 0) {
				dexWriter.write(buf, 0, len);
			}
			dexWriter.close();
			bis.close();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

用到的成員變量如下，

	private String LIB_JAR = "mylib.jar";
	static final int BUF_SIZE = 8 * 1024;
	static final String TAG = "MCL";

重載DexClassLoader方法，

MyDexClassLoader.java

public class MyDexClassLoader extends DexClassLoader {
	public final static String TAG = "MCL";
 
	public MyDexClassLoader(String dexPath, String optimizedDirectory,
			String libraryPath, ClassLoader parent) {
		super(dexPath, optimizedDirectory, libraryPath, parent);
		// TODO Auto-generated constructor stub
	}
 
	@Override
	protected Class<?> findClass(String name) throws ClassNotFoundException {
		// TODO Auto-generated method stub
		Log.i(TAG, "[CL] findCLass: " + name);
		return super.findClass(name);
	}
 
	@Override
	public String findLibrary(String name) {
		// TODO Auto-generated method stub
		Log.i(TAG, "[CL] findLibrary: " + name);
		return super.findLibrary(name);
	}
 
	@Override
	protected URL findResource(String name) {
		// TODO Auto-generated method stub
		Log.i(TAG, "[CL] findResource: " + name);
		return super.findResource(name);
	}
 
	@Override
	protected Enumeration<URL> findResources(String name) {
		// TODO Auto-generated method stub
		Log.i(TAG, "[CL] findResource: " + name);
		return super.findResources(name);
	}
 
	@Override
	protected synchronized Package getPackage(String name) {
		// TODO Auto-generated method stub
		Log.i(TAG, "[CL] getPackage: " + name);
		return super.getPackage(name);
	}
 
	@Override
	public String toString() {
		// TODO Auto-generated method stub
		return "MyDexClassLoader: - " + super.toString();
	}
 
	@Override
	public URL getResource(String resName) {
		// TODO Auto-generated method stub
		Log.i(TAG, "[CL] getResource: " + resName);
		return super.getResource(resName);
	}
 
	@Override
	public Enumeration<URL> getResources(String resName) throws IOException {
		// TODO Auto-generated method stub
		Log.i(TAG, "[CL] getResource: " + resName);
		return super.getResources(resName);
	}
 
	@Override
	public InputStream getResourceAsStream(String resName) {
		// TODO Auto-generated method stub
		Log.i(TAG, "[CL] getResourceAsStream: " + resName);
		return super.getResourceAsStream(resName);
	}
 
	@Override
	public Class<?> loadClass(String className) throws ClassNotFoundException {
		// TODO Auto-generated method stub
		Log.i(TAG, "[CL] loadClass: " + className);
		return super.loadClass(className);
	}
 
	@Override
	protected Class<?> loadClass(String className, boolean resolve)
			throws ClassNotFoundException {
		// TODO Auto-generated method stub
		// Log.i(TAG, String.format("[CL] loadClass: Name - %s, resolved - %b",
		// className, resolve));
		return super.loadClass(className, resolve);
	}
 
	@Override
	protected Package[] getPackages() {
		// TODO Auto-generated method stub
		Log.i(TAG, "[CL] getPackages");
		return super.getPackages();
	}
 
	@Override
	protected Package definePackage(String name, String specTitle,
			String specVersion, String specVendor, String implTitle,
			String implVersion, String implVendor, URL sealBase)
			throws IllegalArgumentException {
		// TODO Auto-generated method stub
		Log.i(TAG, "[CL] definePackage: " + name);
		return super.definePackage(name, specTitle, specVersion, specVendor,
				implTitle, implVersion, implVendor, sealBase);
	}
 
	@Override
	public void setClassAssertionStatus(String cname, boolean enable) {
		// TODO Auto-generated method stub
		Log.i(TAG, String.format(
				"[CL] setClassAssertionStatus: Name - %s, enable - %b", cname,
				enable));
		super.setClassAssertionStatus(cname, enable);
	}
 
	@Override
	public void setPackageAssertionStatus(String pname, boolean enable) {
		// TODO Auto-generated method stub
		Log.i(TAG, String.format(
				"[CL] setPackageAssertionStatus: Name - %s, enable - %b",
				pname, enable));
		super.setPackageAssertionStatus(pname, enable);
	}
 
	@Override
	public void setDefaultAssertionStatus(boolean enable) {
		// TODO Auto-generated method stub.
		Log.i(TAG, "setDefaultAssertionStatus: " + enable);
		super.setDefaultAssertionStatus(enable);
	}
 
	@Override
	public void clearAssertionStatus() {
		// TODO Auto-generated method stub
		Log.i(TAG, "clearAssertionStatus");
		super.clearAssertionStatus();
	}

其只是簡單的重載了父類的方法，並輸出了log。

加載代碼如下，

private void testDynamicLoad() {
		Log.i(TAG, "$$$$$$$$$$$$$$$This is my test$$$$$$$$$$$$$$$");
		ClassLoader parentcl = getClassLoader();
		File libFile = new File(getDir("dex", Context.MODE_PRIVATE), LIB_JAR);
		copy2SDCard(libFile);
		String dexLibPath = getDir("outdex", Context.MODE_PRIVATE)
				.getAbsolutePath();
		MyDexClassLoader cl = new MyDexClassLoader(libFile.getAbsolutePath(),
				dexLibPath, null, parentcl);
		try {
			Class dtClz = cl.loadClass("paul.example.thirddex.Lib");
			ILib lib = (ILib) dtClz.newInstance();
			lib.sayHello();
 
		} catch (ClassNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InstantiationException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
 
	}

MainActivity代碼如下，

public class MainActivity extends ActionBarActivity {
	Button button;
	private String LIB_JAR = "mylib.jar";
	static final int BUF_SIZE = 8 * 1024;
	static final String TAG = "MCL";
 
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		button = (Button) findViewById(R.id.button1);
		button.setOnClickListener(new OnClickListener() {
			@Override
			public void onClick(View v) {
				// TODO Auto-generated method stub
				testDynamicLoad();
			}
		});
	}
 
	@SuppressLint("NewApi")
	private void testDynamicLoad() {
		Log.i(TAG, "$$$$$$$$$$$$$$$This is my test$$$$$$$$$$$$$$$");
		ClassLoader parentcl = getClassLoader();
		File libFile = new File(getDir("dex", Context.MODE_PRIVATE), LIB_JAR);
		copy2SDCard(libFile);
		String dexLibPath = getDir("outdex", Context.MODE_PRIVATE)
				.getAbsolutePath();
		MyDexClassLoader cl = new MyDexClassLoader(libFile.getAbsolutePath(),
				dexLibPath, null, parentcl);
		try {
			Class dtClz = cl.loadClass("paul.example.thirddex.Lib");
			ILib lib = (ILib) dtClz.newInstance();
			lib.sayHello();
 
		} catch (ClassNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InstantiationException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
 
	}
 
	private void copy2SDCard(File dexInternalStoragePath) {
		BufferedInputStream bis;
		try {
			bis = new BufferedInputStream(getAssets().open(LIB_JAR));
			OutputStream dexWriter = new BufferedOutputStream(
					new FileOutputStream(dexInternalStoragePath));
			byte[] buf = new byte[BUF_SIZE];
			int len;
			while ((len = bis.read(buf, 0, BUF_SIZE)) > 0) {
				dexWriter.write(buf, 0, len);
			}
			dexWriter.close();
			bis.close();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
 
	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}
 
	@Override
	public boolean onOptionsItemSelected(MenuItem item) {
		// Handle action bar item clicks here. The action bar will
		// automatically handle clicks on the Home/Up button, so long
		// as you specify a parent activity in AndroidManifest.xml.
		int id = item.getItemId();
		if (id == R.id.action_settings) {
			return true;
		}
		return super.onOptionsItemSelected(item);
	}
}

在運行程序之前，一定要先把Lib.java文件刪掉，然後運行，運行結果，

若工程中沒有刪除Lib.java造成的影響。則運行結果下圖，

分析：

若工程中沒有刪除Lib.java文件，則該應用的ClassLoader會包含該類，MyDexClassLoader加載Lib Class的時候，會先通過parent ClassLoader來加載該類，也就是該應用的ClassLoade加載，若Lib.java未刪除，則parent ClassLoader會加載成功。

若刪除了Lib.java文件，則parent ClassLoader未加載成功，會用MyDexClassLoader來加載。這也就解釋了上面兩個結果。

另外，若沒有定義接口，則在調用該類方法的時候，必須通過反射來調用，比較麻煩，而定義了接口，就可以轉化爲接口來調用。

有幾項需要注意，

1. 在打包java文件的時候，一定注意不要將接口文件打包進來

2. dexcalssloader的優化目錄一定要是程序私有目錄

3. 若該工程內包含Lib.java，則一定要刪除。也可以新建一個Android項目，包含Lib.java，然後在另一個項目中動態加載Lib。

工程下載地址

http://download.csdn.net/detail/u014088294/9251541

fabowang

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