动态加载DEX

摘要：一什么是动态加载为什么要动态加载动态加载就是用到的时候再去加载，也叫懒加载，也就意味着用不到的时候是不会去加载的。

动态加载就是用到的时候再去加载，也叫懒加载，也就意味着用不到的时候是不会去加载的。

jar是JDK提供的，dx是Android SDK提供，只要配置好环境变量，可在任意位置执行
我是在debug中执行，方便使用
jar -cvf cn 生成jar包
dx --dex --output=target.jar source.jar 生成包含dex的jar包
注意：把dex放入assets后,把DynamicDex.java删掉或改名，否则加载的是它，而不是dex中的类

    public class DexUtil {
    public static void loadDexClass(final Context context, final String dexName, final Handler handler) {
        new AsyncTask() {
            @Override
            protected String doInBackground(Void... params) {

                File cacheFile = getCacheDir(context.getApplicationContext());
                String internalPath = cacheFile.getAbsolutePath() + File.separator + dexName;
                Log.v("DexUtil", "internalPath = " + internalPath);
                File desFile = new File(internalPath);
                if (!desFile.exists()) {
                    try {
                        if (!desFile.exists()) {
                            desFile.createNewFile();
                            copyFiles(context, dexName, desFile);
                        }
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
                return internalPath;
            }

            @Override
            protected void onPostExecute(String internalPath) {
                super.onPostExecute(internalPath);
                File dexOutputDir = context.getDir("dex", 0);
                File soOutputDir = context.getDir("lib", 0);
                String librarySerachPath = soOutputDir.getAbsolutePath().replace("app_lib", "lib");
                Log.v("DexUtil", "librarySerachPath = " + librarySerachPath);
                DexClassLoader dexClassLoader = new DexClassLoader(internalPath, dexOutputDir.getAbsolutePath(),
                        librarySerachPath, context.getClassLoader());//ClassLoader.getSystemClassLoader().getParent());
                Message msg = new Message();
                msg.obj = dexClassLoader;
                handler.sendMessage(msg);
            }
        }.execute();

    }


    public static void copyFiles(Context context, String fileName, File desFile) {
        InputStream in = null;
        OutputStream out = null;
        try {
            in = context.getApplicationContext().getAssets().open(fileName);
            out = new FileOutputStream(desFile.getAbsolutePath());
            byte[] bytes = new byte[1024];
            int i;
            while ((i = in.read(bytes)) != -1) {
                out.write(bytes, 0, i);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (in != null) {
                    in.close();
                }
                if (out != null) {
                    out.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }

        }
    }

    public static boolean hasExternalStorage() {
        return Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED);
    }

    /**
     * 获取缓存路径
     *
     * @param context
     * @return 返回缓存文件路径
     */
    public static File getCacheDir(Context context) {
        File cache;
        if (hasExternalStorage()) {
            cache = context.getExternalCacheDir();
        } else {
            cache = context.getCacheDir();
        }
        if (!cache.exists())
            cache.mkdirs();
        return cache;
    }

}

Dex加载步骤:
1)把assets中的dex的jar拷贝到met目录下
文件拷贝需要添加权限

2）利用DexClassLoader加载Dex,把dex放入到相关目录上


注意:文件的拷贝需要放在子线程中运行
3.利用反射加载

public class MainActivity extends AppCompatActivity {

    public static DexClassLoader mapLoader;
    TextView tv_hello;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv_hello = (TextView) this.findViewById(R.id.tv_hello);
        DexUtil.loadDexClass(this, "dex_dynamic.jar", dexHandler);

    }

    private Handler dexHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            mapLoader = (DexClassLoader) msg.obj;
            try {
                Class DynamicDex_Class = mapLoader.loadClass("cn.liufei.dynamicdex.DynamicDex");
                Constructor DynamicDex_Cons = DynamicDex_Class.getConstructor(null);
                Object DynamicDex_Obj = DynamicDex_Cons.newInstance();
                Method test_meth = DynamicDex_Class.getMethod("test", null);
                String result = (String) test_meth.invoke(DynamicDex_Obj, null);
                tv_hello.setText(result);
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            } catch (NoSuchMethodException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            } catch (InstantiationException e) {
                e.printStackTrace();
            } catch (InvocationTargetException e) {
                e.printStackTrace();
            }

        }
    };
}

Demo地址：链接: https://pan.baidu.com/s/1kVMQ6jX 密码: i4vp