Android的ClassLoader体系
在Android中可以跟java一样实现动态加载jar,但是Android使用Dalvik VM,不能直接加载java打包jar的byte code,需要通过dx工具来优化Dalvik byte code
Android在API中给出可动态加载的有:DexClassLoader 和 PathClassLoader。
DexClassLoader:可加载jar、apk和dex,可以从SD卡中加载
PathClassLoader:只能加载已安装到系统中(即/data/app目录下)的apk文件
为什么PathClassLoader只能加载apk的文件?
从上图可以明显知道他们都继承BaseDexClassLoader 在看看他们源码有什么区别
// DexClassLoader.java public class DexClassLoader extends BaseDexClassLoader { public DexClassLoader(String dexPath, String optimizedDirectory, String libraryPath, ClassLoader parent) { super(dexPath, new File(optimizedDirectory), libraryPath, parent); } } // PathClassLoader.java public class PathClassLoader extends BaseDexClassLoader { public PathClassLoader(String dexPath, ClassLoader parent) { super(dexPath, null, null, parent); } public PathClassLoader(String dexPath, String libraryPath, ClassLoader parent) { super(dexPath, null, libraryPath, parent); } }
主要的区别在于PathClassLoader的optimizedDirectory参数只能是null,那么optimizedDirectory是做什么用的呢?需要去父类看BaseDexClassLoader
public BaseDexClassLoader(String dexPath, File optimizedDirectory, String libraryPath, ClassLoader parent) { super(parent); this.originalPath = dexPath; this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory); }
他们的父类构造方法中 是new 一个DexPathList实例把optimizedDirectory参数传入
public DexPathList(ClassLoader definingContext, String dexPath, String libraryPath, File optimizedDirectory) { …… this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory); } private static Element[] makeDexElements(ArrayList<File> files, File optimizedDirectory) { ArrayList<Element> elements = new ArrayList<Element>(); for (File file : files) { ZipFile zip = null; DexFile dex = null; String name = file.getName(); if (name.endsWith(DEX_SUFFIX)) { dex = loadDexFile(file, optimizedDirectory); } else if (name.endsWith(APK_SUFFIX) || name.endsWith(JAR_SUFFIX) || name.endsWith(ZIP_SUFFIX)) { zip = new ZipFile(file); } …… if ((zip != null) || (dex != null)) { elements.add(new Element(file, zip, dex)); } } return elements.toArray(new Element[elements.size()]); } private static DexFile loadDexFile(File file, File optimizedDirectory) throws IOException { if (optimizedDirectory == null) { return new DexFile(file); } else { String optimizedPath = optimizedPathFor(file, optimizedDirectory); return DexFile.loadDex(file.getPath(), optimizedPath, 0); } } /** * Converts a dex/jar file path and an output directory to an * output file path for an associated optimized dex file. */ private static String optimizedPathFor(File path, File optimizedDirectory) { String fileName = path.getName(); if (!fileName.endsWith(DEX_SUFFIX)) { int lastDot = fileName.lastIndexOf("."); if (lastDot < 0) { fileName += DEX_SUFFIX; } else { StringBuilder sb = new StringBuilder(lastDot + 4); sb.append(fileName, 0, lastDot); sb.append(DEX_SUFFIX); fileName = sb.toString(); } } File result = new File(optimizedDirectory, fileName); return result.getPath(); }
optimizedDirectory是用来缓存我们需要加载的dex文件的,并创建一个DexFile对象,如果它为null,那么会直接使用dex文件原有的路径来创建DexFile 对象。
optimizedDirectory必须是一个内部存储路径,无论哪种动态加载,加载的可执行文件一定要存放在内部存储。DexClassLoader可以指定自己的optimizedDirectory,所以它可以加载外部的dex,因为这个dex会被复制到内部路径的optimizedDirectory;而PathClassLoader没有optimizedDirectory,所以它只能加载存在系统中已经安装过的apk里面的内部的dex
怎么缓存经过优化的classes(odex文件)?
使用Context.getDir(String, int)方法可以创建一个这样的目录,例如:
File dexOutputDir = context.getDir("dex", 0);
DexClassLoader
public DexClassLoader (String dexPath, String dexOutputDir, String libPath, ClassLoader parent)
dexPath:dex文件路径列表,多个路径使用”:”分隔
dexOutputDir:经过优化的dex文件(odex)文件输出目录
libPath:动态库路径(将被添加到app动态库搜索路径列表中)
parent:这是一个ClassLoader,这个参数的主要作用是保留java中ClassLoader的委托机制(优先父类加载器加载classes,由上而下的加载机制,防止重复加载类字节码)
PathClassLoader
PathClassLoader提供两个常用构造方法
public PathClassLoader (String path, ClassLoader parent) public PathClassLoader (String path, String libPath, ClassLoader parent)
path:文件或者目录的列表
libPath:包含lib库的目录列表
parent:父类加载器
PathClassLoader提供一个简单的ClassLoader实现,可以操作在本地文件系统的文件列表或目录中的classes,但不可以从网络中加载classes。
DexClassloader
编写接口:Dynamic
package com.smilegames.dynamic.interfaces; public interface Dynamic { public String helloWorld(); public String smileGames(); public String fyt(); }
编写实现类:DynamicTest
package com.smilegames.dynamic.impl; import com.smilegames.dynamic.interfaces.Dynamic; public class DynamicTest implements Dynamic { @Override public String helloWorld() { return "Hello Word!"; } @Override public String smileGames() { return "Smile Games"; } @Override public String fyt() { return "fengyoutian"; } }
打包并编译成dex
将接口打包成jar:dynamic.jar(只打包这Dynamic.java这一个接口)
将实现类打包成jar:dynamic_test.jar(只打包DynamicTest.java这一个实现类)
将打包后的实现类(dynamic_test.jar)编译成dex:dynamic_impl.jar
1、将dynamic_test.jar拷贝到SDK安装目录android-sdk-windowsplatform-tools下(ps:如果platform-tools没有dx.bat,可拷贝到build-tools目录下有dx.bat的子目录)
2、执行以下命令:
dx --dex --output=dynamic_impl.jar dynamic_test.jar
3、将dynamic.jar引入测试实例
4、将dynamic_impl.jar放到模拟器或真机的sdcard
修改onCreate例子
@Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); // 或许activity按钮 helloWorld = (Button) findViewById(R.id.helloWorld); smileGames = (Button) findViewById(R.id.smileGames); fyt = (Button) findViewById(R.id.fyt); /*使用DexCkassLoader方式加载类*/ // dex压缩文件的路径(可以是apk,jar,zip格式) String dexPath = Environment.getExternalStorageDirectory().toString() + File.separator + "dynamic_impl.jar"; // dex解压释放后的目录 //String dexOutputDirs = Environment.getExternalStorageDirectory().toString(); File dexOutputDir = context.getDir("dex", 0); DexClassLoader dexClassLoader = new DexClassLoader(dexPath,dexOutputDir.getAbsolutePath(),null,getClassLoader()); //主意这里目录必须是应用内的目录 //如果是应用外的目录例如String dexOutputDirs = Environment.getExternalStorageDirectory().toString()+"XXX"; //会报:java.lang.IllegalArgumentException: Optimized data directory /storage/sdcard0 is not owned by the current user. Shared storage cannot protect your application from code injection attacks //原因之前都已经讲过 // 定义DexClassLoader // 第一个参数:是dex压缩文件的路径 // 第二个参数:是dex解压缩后存放的目录 // 第三个参数:是C/C++依赖的本地库文件目录,可以为null // 第四个参数:是上一级的类加载器 DexClassLoader dexClassLoader = new DexClassLoader(dexPath,dexOutputDirs,null,getClassLoader()); Class libProvierClazz = null; // 使用DexClassLoader加载类 try { libProvierClazz = dexClassLoader.loadClass("com.smilegames.dynamic.impl.DynamicTest"); // 创建dynamic实例 dynamic = (Dynamic) libProvierClazz.newInstance(); } catch (Exception e) { e.printStackTrace(); } helloWorld.setOnClickListener(new HelloWorldOnClickListener()); smileGames.setOnClickListener(new SmileGamesOnClickListener()); fyt.setOnClickListener(new FytOnClickListener()); } private final class HelloWorldOnClickListener implements View.OnClickListener { @Override public void onClick(View v) { if (null != dynamic) { Toast.makeText(getApplicationContext(), dynamic.helloWorld(), 1500).show(); } else { Toast.makeText(getApplicationContext(), "类加载失败", 1500).show(); } } } private final class SmileGamesOnClickListener implements View.OnClickListener { @Override public void onClick(View v) { if (null != dynamic) { Toast.makeText(getApplicationContext(), dynamic.smileGames(), 1500).show(); } else { Toast.makeText(getApplicationContext(), "类加载失败", 1500).show(); } } }