動態加載DEX

一.什麼是動態加載？爲何要動態加載？

動態加載就是用到的時候再去加載，也叫懶加載，也就意味着用不到的時候是不會去加載的。

二.編寫Demo

1.利用jar,dx 建立dex

1)建立DynamicDex.java,生成DynamicDex.class

2)生成dex

jar是JDK提供的，dx是Android SDK提供，只要配置好環境變量，可在任意位置執行
我是在debug中執行，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後,把DynamicDex.java刪掉或更名，不然加載的是它，而不是dex中的類

2.加載Dex

public class DexUtil {
    public static void loadDexClass(final Context context, final String dexName, final Handler handler) {
        new AsyncTask<Void, Void, String>() {
            @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);
    }

    /**
     * 獲取緩存路徑
     *
     * @param context
     * @return 返回緩存文件路徑
     */
    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加載步驟:
1)把assets中的dex的jar拷貝到met目錄下
文件拷貝須要添加權限

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

2）利用DexClassLoader加載Dex,把dex放入到相關目錄上


注意:文件的拷貝須要放在子線程中運行
3.利用反射加載

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：連接: https://pan.baidu.com/s/1kVMQ6jX 密碼: i4vp