1.思路：

• 1.1 通過getPackageArchiveInfo方法獲取sd卡的未安裝的APK信息和Activity
• 1.2 獲取到插件Activity的信息之后，通過反射得到插件Activity的實例
• 1.3 獲取到插件Activity的實例后通過構造函數將宿主的上下文傳遞到插件APP
• 1.4 在反射調用插件activity的onCreate方法

2.宿主代碼

package com.lazy.dex;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.pm.PackageInfo;
import android.os.Bundle;
import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import dalvik.system.DexClassLoader;
import java.io.File;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;

public class MainActivity extends AppCompatActivity {

  private String mClass;
  String dexPath = Environment.getExternalStorageDirectory().toString() + File.separator + "a";

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    findViewById(R.id.bt).setOnClickListener(new OnClickListener() {
      @Override
      public void onClick(View v) {
        launchTargetActivity();
      }
    });
  }


  @SuppressLint("NewApi")
  protected void launchTargetActivity() {
    //根據sd卡的路徑獲取未安裝的apk的信息
    PackageInfo packageInfo = getPackageManager().getPackageArchiveInfo(dexPath, 1);
    if ((packageInfo.activities != null)
        && (packageInfo.activities.length > 0)) {
      String activityName = packageInfo.activities[0].name;//獲取插件中第一個activity
      mClass = activityName;
      launchTargetActivity(mClass);
    }
  }

  @SuppressLint("NewApi")
  protected void launchTargetActivity(final String className) {
    Log.e("MainActivity", "start launchTargetActivity, className=" + className);
    File dexOutputDir = this.getDir("dex", 0);
    final String dexOutputPath = dexOutputDir.getAbsolutePath();
    ClassLoader localClassLoader = ClassLoader.getSystemClassLoader();
    DexClassLoader dexClassLoader = new DexClassLoader(dexPath,
        dexOutputPath, null, localClassLoader);
    try {
      Class<?> localClass = dexClassLoader.loadClass(className);//創建插件Activity的實例
      //調用插件Activity的構造函數，將當前activity的上下文傳遞到插件activity中去
      Constructor<?> localConstructor = localClass.getConstructor(new Class[]{Activity.class});
      Object instance = localConstructor.newInstance(new Object[]{this});
      Log.e("MainActivity", "instance = " + instance);
      Method onCreate = localClass.getDeclaredMethod("onCreate", new Class[]{Bundle.class});
      onCreate.setAccessible(true);
      Bundle bundle = new Bundle();
      onCreate.invoke(instance, bundle);//調用插件activity的onCreate方法
    } catch (Exception e) {
      Log.e("MainActivity", e.getMessage());
    }
  }


}

3.APP插件代碼

package com.Company.Demo;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.Toast;

/**
 * 項目名稱：Demo1
 * 類描述：
 * 創建人：Administrator
 * 創建時間：2017/9/20 17:24
 * 修改人：Administrator
 * 修改時間：2017/9/20 17:24
 * 修改備注：
 * 聯系方式：906514731@qq.com
 */
public class TestActivity extends Activity implements OnClickListener {

  /**
   * 宿主的上下文引用
   */
  private Activity otherActivity;

  /**
   * 無參構造函數
   */
  public TestActivity() {
    super();
  }

  /**
   * 有參構造函數
   * @param context
   */
  public TestActivity(Activity context) {
    super();
    otherActivity = context;
    Log.e("TestActivity", otherActivity.toString() );
  }

  @Override
  public void onCreate(Bundle savedInstanceState) {
    if (otherActivity != null) {
      // 使用TestHallActivity的上下文引用創建View并添加到根視圖
      Button button = new Button(otherActivity);
      button.setText("我是插件TestActivity");
      button.setOnClickListener(this);

      LinearLayout root = new LinearLayout(otherActivity);

      root.addView(button);

      //使用宿主的上線文就不能使用插件的資源了
      otherActivity.setContentView(root);

    } else {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
    }
  }

  /**
   * 返回當前Activity的描述信息
   * @return
   */
  private String getRoomMessage() {
    return "我是插件TestActivity";
  }

  @Override
  public void onClick(View v) {

    if (otherActivity != null) {
      // Toast.makeText(this, "This is Room A!", Toast.LENGTH_SHORT).show();是不合適的調用
      Toast.makeText(otherActivity, "我是插件TestActivity", Toast.LENGTH_SHORT).show();
    } else {
      Toast.makeText(this, "我是插件TestActivity", Toast.LENGTH_SHORT).show();
    }

  }

}