This Handler class should be static or leaks might occur (com.test.test3.ui.MainActivity.1)

Question

I am new to android and i try to develop a system but when i finish code the handler show this warning

below show the code after I edit, the handler in event ontounch show the warning handler cannot be resolved. I try putting // to ignore the handler at i try run the application and its result in force close.

public class MainActivity extends Activity {



protected static final int STOP = 100;
ImageView iv;
private ProgressBar pb;
LinearLayout ll;
private AnimationDrawable anim;
ScrollView sv;
private SQLiteDatabase db;
private boolean flagscanning = false;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ll = new LinearLayout(this);
    new HandlerClass(this);

            db = SQLiteDatabase.openDatabase(Environment.getExternalStorageDirectory()+"/antivirus.sqlite", null, SQLiteDatabase.OPEN_READONLY);  
            iv = (ImageView) this.findViewById(R.id.imageView1);
                    //扫描病毒进度条
            pb = (ProgressBar) this.findViewById(R.id.progressBar1);
            ll = (LinearLayout) this.findViewById(R.id.ll);
                    //设置ImageView背景资源为动画文件
            iv.setBackgroundResource(R.drawable.bg);
                    //sv用来显示病毒的扫描结果
            sv = (ScrollView) this.findViewById(R.id.scrollView1);
            anim = (AnimationDrawable) iv.getBackground();
}

private static class HandlerClass extends Handler{
    private final WeakReference<MainActivity> mTarget;
    public HandlerClass(MainActivity context){
        mTarget = new WeakReference<MainActivity>((MainActivity) context);
    }

    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
        MainActivity target = mTarget.get();
         if(msg.what==STOP){
             target.ll.removeAllViews();
             //anim.stop();

             }
         String str = (String) msg.obj; 
         TextView tv = new TextView(target);
         tv.setText(str);
         target.ll.setOrientation(LinearLayout.VERTICAL);
         target.ll.addView(tv);
         //sv.scrollBy(0, 20);

        System.out.println(str);

    }
};


@Override
public boolean onTouchEvent(MotionEvent event) {
    //如果程序正在杀毒过程中，拒绝再次启动杀毒线程
    if(flagscanning){
        return false;
    }

    //如果用户触摸屏幕，则开启杀毒线程  
    if (event.getAction() == MotionEvent.ACTION_UP) {
        flagscanning= true;
        anim.start();
        new Thread() {
            public void run() {
                // 获取每一个应用程序的签名，签名须与数据库的签名想比较
                List<PackageInfo> infos = getPackageManager()
                        .getInstalledPackages(PackageManager.GET_UNINSTALLED_PACKAGES | PackageManager.GET_SIGNATURES);
                //设置进度条的扫描范围
                pb.setMax(infos.size());
                int total = 0;
                int virustotal = 0;//设置初始病毒数为0
                for (PackageInfo info : infos) {
                    total++;
                    try {
                        sleep(20);//只为便于观察扫描效果和进度，无实质作用
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    Message msg = Message.obtain();
                    msg.obj = "正在扫描" + info.packageName;
                    _handler.sendMessage(msg);_
                    Signature[] signs = info.signatures;
                    String str = signs[0].toCharsString();

                    String md5 = MD5Encoder.encode(str);
                    //将应用程序签名与数据库中保存的签名进行比较，如果相一致，则使病毒数加1，并通过handler在界面显示病毒包名
                    Cursor cursor = db.rawQuery("select desc from datable where md5=?",new String[] { md5 });
                    if (cursor.moveToFirst()) {
                        String desc = cursor.getString(0);
                        msg = Message.obtain();
                        msg.obj = info.packageName + ": " + desc;
                        _handler.sendMessage(msg);_
                        virustotal++;
                    }
                    cursor.close();
                    pb.setProgress(total);

                }
                Message msg = Message.obtain();
                msg.what = STOP;
                msg.obj = "扫描完毕 ,共发现" + virustotal + "个病毒";
                _handler.sendMessage(msg);_
                flagscanning = false;
                pb.setProgress(0);
            };
        }.start();
    }
    return super.onTouchEvent(event);
}

@Override
protected void onDestroy() {
    if (db.isOpen())
        db.close();
    super.onDestroy();
}

@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;
}
}

Answer 1

Make your handler a static class.

The warning is a lint warning. You can disable the warning but its a useful info

Here's a list of Lint Check

http://tools.android.com/tips/lint-checks

Quoting from the source @

http://android-developers.blogspot.in/2009/01/avoiding-memory-leaks.html

Avoid non-static inner classes in an activity if you don't control their life cycle, use a static inner class and make a weak reference to the activity inside.

The solution to this issue is to use a static inner class with a WeakReference to the outer class, as done in ViewRoot and its W inner class for instance.

Also check this discussion on android developers group. Check the solution by Romain Guy

https://groups.google.com/forum/#!topic/android-developers/1aPZXZG6kWk

Example from Romain Guy's solution from the above link

 class OuterClass { 
 class InnerClass { 
  private final WeakReference<OuterClass> mTarget; 

   InnerClass(OuterClass target) { 
    mTarget = new WeakReference<OuterClass>(target); 
  } 

  void doSomething() { 
  OuterClass target = mTarget.get(); 
  if (target != null) target.do(); 
   }

Edit:

Example:

public class MainActivity extends Activity {

      LinearLayout ll;
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ll = new LinearLayout(this);
        new HandlerClass(this);
    }
       private static class HandlerClass extends Handler{
           private final WeakReference<MainActivity> mTarget; 
        public HandlerClass(MainActivity context)
        {
             mTarget = new WeakReference<MainActivity>((MainActivity) context);

        }

            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
                MainActivity target = mTarget.get(); 
                if (target != null) 
                 if(msg.what==1){
                     target.ll.removeAllViews();
                    // anim.stop();

                     }
                 String str = (String) msg.obj;
                 TextView tv = new TextView(target);
                 tv.setText(str);
                 target.ll.setOrientation(LinearLayout.VERTICAL);
                 target.ll.addView(tv);
                 //sv.scrollBy(0, 20);

                System.out.println(str);

            }

        };
}

Correct me if the above is wrong or has some issues.

You can also check this blog by Alex Lockwood

http://www.androiddesignpatterns.com/2013/01/inner-class-handler-memory-leak.html