Android开发中使用static变量应该注意的问题

 package com.highxin.launcher01;

 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;

 import android.app.Activity;
 import android.content.ComponentName;
 import android.content.ContentResolver;
 import android.content.Context;
 import android.content.Intent;
 import android.content.pm.ApplicationInfo;
 import android.content.pm.PackageInfo;
 import android.content.pm.PackageManager;
 import android.content.pm.ResolveInfo;
 import android.database.Cursor;
 import android.graphics.drawable.Drawable;
 import android.net.Uri;
 import android.os.Bundle;
 import android.os.Handler;
 import android.os.Message;
 import android.provider.ContactsContract;
 import android.provider.ContactsContract.CommonDataKinds;
 import android.provider.ContactsContract.PhoneLookup;
 import android.provider.MediaStore;
 import android.util.Log;
 import android.view.KeyEvent;
 import android.view.LayoutInflater;
 import android.view.View;
 import android.view.ViewGroup;
 import android.view.Window;
 import android.widget.AdapterView;
 import android.widget.AdapterView.OnItemClickListener;
 import android.widget.BaseAdapter;
 import android.widget.GridView;
 import android.widget.ImageView;
 import android.widget.TextView;

 public class MainActivity extends Activity implements Runnable,OnItemClickListener{
     String name = "";
     String phoneNO = "";
     public static  List<Map<String, Object>>  contacts_list = new ArrayList<Map<String, Object>>();
     public static  List<Map<String, Object>>  gv_applist = new ArrayList<Map<String,Object>>();
     public static boolean switch_flag = false;
     public static boolean click_flag = false;
     public static boolean firstload_flag = false;//判断桌面是否是第一次加载
     public static List<Integer> click_position = new ArrayList<Integer>();
     private Intent serviceIntent;

     public static List<ResolveInfo> appInfos;
     public static List<ResolveInfo> showappInfos;
     public static  List<Map<String, Object>>  lv_applist = new ArrayList<Map<String,Object>>();
     public static  List<Map<String, Object>>  lv_addapplist = new ArrayList<Map<String,Object>>();
     public static  List<Map<String, Object>>  lv_delapplist = new ArrayList<Map<String,Object>>();

     static int []clickinfo;

     static GridView gv_app;

     String []appname ={"联系人","相机","电话","收音机","手电筒","更多"};
     int []appicon = {R.drawable.contact,R.drawable.camera,R.drawable.dial,R.drawable.fm,R.drawable.light_off,R.drawable.shop};

     private Handler mHandler = new Handler() {

         @Override
         public void handleMessage(Message msg) {
             super.handleMessage(msg);

              showappInfos = appInfos;

              for(int i=0; i<appInfos.size();i++) {
                  if((appInfos.get(i).loadLabel(getPackageManager()).equals("相机") &&
                      appInfos.get(i).activityInfo.packageName.equals("com.android.camera")) ||
                     (appInfos.get(i).loadLabel(getPackageManager()).equals("收音机") &&
                     appInfos.get(i).activityInfo.packageName.equals("com.miui.fmradio"))  ||
                     (appInfos.get(i).loadLabel(getPackageManager()).equals("老人桌面") &&
                      appInfos.get(i).activityInfo.packageName.equals("com.highxin.launcher01")) ){
                      showappInfos.remove(i);
                  }
              }
              clickinfo = new int[showappInfos.size()];
              gv_app.setAdapter(new GridViewAdapter(MainActivity.this,gv_applist));
         }
     };

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         requestWindowFeature(Window.FEATURE_NO_TITLE);
         setContentView(R.layout.activity_main);
         Drawable drawable_appicon = null;
         String s_appname = null;
         serviceIntent = new Intent(this, LightService.class);
 //        if(!firstload_flag) {
             for(int i=0; i<appname.length;i++) {
                 drawable_appicon = getResources().getDrawable(appicon[i]);
                 s_appname = appname[i];
                 Map<String, Object> map = new HashMap<String, Object>();
                 map.put("appname", s_appname);
                 map.put("drawable", drawable_appicon);

                 gv_applist.add(map);
              }
 //            firstload_flag = true;
 //        }
         gv_app = (GridView) findViewById(R.id.gv_apps);

         gv_app.setOnItemClickListener(this);

         Thread t = new Thread(this);
         t.start();
     }

      private List<Map<String, Object>> fillMaps() {
          List<Map<String, Object>> items = new ArrayList<Map<String, Object>>();

          ContentResolver cr = getContentResolver();
          HashMap<String,ArrayList<String>> hashMap = new HashMap<String,ArrayList<String>>();
          Cursor phone = cr.query(CommonDataKinds.Phone.CONTENT_URI,
            new String[] {
            CommonDataKinds.Phone.CONTACT_ID,
            CommonDataKinds.Phone.DISPLAY_NAME,
            CommonDataKinds.Phone.NUMBER,
            CommonDataKinds.Phone.DATA1
            //CommonDataKinds.StructuredPostal.DATA3,
            },
            null, null, null);
          while (phone.moveToNext()) {
           String contactId = phone.getString(phone.getColumnIndex(CommonDataKinds.Phone.CONTACT_ID));
           String displayName = phone.getString(phone.getColumnIndex(CommonDataKinds.Phone.DISPLAY_NAME));
           String PhoneNumber = phone.getString(phone.getColumnIndex(ContactsContract.CommonDataKinds.Phone.NUMBER));
           String address = phone.getString(phone.getColumnIndex(CommonDataKinds.Phone.DATA1)); 

           //以contactId为主键，把同一人的所有电话都存到一起。
          ArrayList<String> ad = hashMap.get(contactId);
           if(ad == null){
            ad = new ArrayList<String>();
            ad.add(displayName);
            ad.add(PhoneNumber);
            //ad.add(address);

            hashMap.put(contactId, ad);
           }
           else{
            ad.add(PhoneNumber);
           }

          }
          phone.close();  

          ArrayList<String> tmpList;
          String tmpStr = "";
          int k;
          Iterator iter = hashMap.entrySet().iterator();
          while (iter.hasNext()) {
           HashMap.Entry entry = (HashMap.Entry) iter.next();
              Object key = entry.getKey();
              Object val = entry.getValue();

              tmpList = (ArrayList) val;
              tmpStr =  tmpList.get(1);
              //注释掉的为读取多个联系人的方法
 //             for(k = 1; k < tmpList.size(); k++){
 //              tmpStr = tmpStr + tmpList.get(k) + ',' ;
 //             }

              HashMap<String, Object> tmpMap = new HashMap<String, Object>();
              tmpMap.put("name", tmpList.get(0));
              tmpMap.put("phoneNO", tmpStr);
              tmpMap.put("img", R.drawable.image);

              items.add(tmpMap);
          }
          return items;
      }

     class GridViewAdapter extends BaseAdapter{

         LayoutInflater inflater;
         List<Map<String, Object>> gvlist;
         public GridViewAdapter(Context context,List<Map<String, Object>> locallist) {
             inflater = LayoutInflater.from(context);
             gvlist = locallist;
         }
         @Override
         public int getCount() {
             // TODO Auto-generated method stub
             return gvlist.size();
         }

         @Override
         public Object getItem(int position) {
             // TODO Auto-generated method stub
             return gvlist.get(position);
         }

         @Override
         public long getItemId(int position) {
             // TODO Auto-generated method stub
             return position;
         }

         @Override
         public View getView(int position, View convertView, ViewGroup parent) {
             // TODO Auto-generated method stub

                 View view = inflater.inflate(R.layout.gv_item, null);
                 TextView name = (TextView)view.findViewById(R.id.gv_item_appname);
                 ImageView iv = (ImageView)view.findViewById(R.id.gv_item_icon);
                 name.setText((CharSequence) gvlist.get(position).get("appname"));
                 iv.setImageDrawable((Drawable) gvlist.get(position).get("drawable"));

             return view;
         }

     }

     @Override
     protected void onResume() {
         super.onResume();
          gv_app.setAdapter(new GridViewAdapter(MainActivity.this,gv_applist));
     }

     @Override
     public void onItemClick(AdapterView<?> arg0, View arg1, int arg2, long arg3) {
         // TODO Auto-generated method stub
         switch(arg2) {

             case 0:{
                     contacts_list = fillMaps();
                     Intent contact  = new Intent(MainActivity.this, Contacts.class);
                     startActivity(contact);
                     }break;
             case 1:{
                Intent imageCaptureIntent = new Intent("android.media.action.STILL_IMAGE_CAMERA");
                startActivityForResult(imageCaptureIntent,1);
             }break;
             case 2:{
                 Intent i = new Intent();
                 String packageName = "com.android.contacts";
                 String mainActivityName = "com.android.contacts.activities.TwelveKeyDialer";
                 i.setComponent(new ComponentName(packageName,mainActivityName));
                 startActivity(i);
             }break;
             case 3:{
                 Intent intent = new Intent();
                 intent.setPackage("com.miui.fmradio");
                 startActivityForResult(intent, 3);
                 }break;
             case 4:{
                     switch_flag = !switch_flag;
                     if(switch_flag) {
                         startService(serviceIntent);
                          ImageView iv = (ImageView) arg1.findViewById(R.id.gv_item_icon);
                          iv.setImageResource(R.drawable.light);

                     }else {
                         stopService(serviceIntent);
                          ImageView iv = (ImageView) arg1.findViewById(R.id.gv_item_icon);
                          iv.setImageResource(R.drawable.light_off);
                     }
                 }break;
             case 5:{
                 Intent applist  = new Intent(MainActivity.this, AppList.class);
                 startActivityForResult(applist,5);
                 }break;
             default:{
                 Intent i = new Intent();
                 String packageName = (String) MainActivity.gv_applist.get(arg2).get("pkgname");
                 String mainActivityName = (String) MainActivity.gv_applist.get(arg2).get("activityname");
                 i.setComponent(new ComponentName(packageName,mainActivityName));
                 startActivity(i);

             }break;
         }
     }

     @Override
     public void run() {

          appInfos = getResolveInfoLists();
          mHandler.sendEmptyMessage(0);
     }

     /**
      * @Title: getResolveInfoLists
      * @Description: 它是通过解析< Intent-filter>标签得到有 　　< action
      *               android:name=”android.intent.action.MAIN”/> 　　< action
      *               android:name=”android.intent.category.LAUNCHER”/>
      * @param ：
      * @return List<ResolveInfo>
      * @throws
      */
     private List<ResolveInfo> getResolveInfoLists() {

         // TODO Auto-generated method stub
         PackageManager packageManager = this.getPackageManager();

         Intent intent = new Intent(Intent.ACTION_MAIN);
         intent.addCategory(Intent.CATEGORY_LAUNCHER);

         return packageManager.queryIntentActivities(intent, 0);
     }

     @Override
     protected void onDestroy() {
         // TODO Auto-generated method stub
         super.onDestroy();
 //        gv_applist.clear();
     }

 //    @Override
 //    public boolean onKeyDown(int keyCode, KeyEvent event) {
 //        if (keyCode == KeyEvent.KEYCODE_BACK){
 //            return true;
 //        }
 //        if(keyCode == KeyEvent.KEYCODE_HOME) {
 //            gv_applist.clear();
 //        }
 //
 //    return super.onKeyDown(keyCode, event);
 //
 //    }
 }

每次按返回键，重新进入程序gv_applist.add(map)，会在原来的基础上不断增加。

找到一篇文章是描述的大概也是这个问题：

突然想到了一个问题，因为我在这两个Activity中使用了不一样的结束方式，一个为System.exit(0);一种为finishi();感觉可能是这两个方式造成的，后来经过测试，确实是这两个方式造成的，如果是使用System.exit（0）；的方式的话，等于把这个Acitvity结束了，因此他所操作过的数据都不会再存在了，系统也许认为没有存在的必要了吧，使用finishi();的话，虽然也是退出，但是这个并不是释放资源，只是把当前的Activity推向后台，不再显示，但是他不释放资源，具体资源什么时候释放，由系统决定，当然System.exit（0）；这种形式，也并不是真正的退出系统，因为我们还可能有别的Activity在运行，但是他确实把资源释放了！这个是为什么，不太明白！

的确重写onDestroy(),调用finish方法还是有这个问题，但是调用System.exit(0)就不存在这个问题啦。如果仅仅就是解决这个问题，当然可以继续坚持使用static变量，要么在destroy方法中对gv_applist清零,要么设置变量判断是不是第一次启动。

@Override
 protected void onDestroy() {
  // TODO Auto-generated method stub
  super.onDestroy();
//  System.exit(0);
  finish();
  
//  gv_applist.clear();
 }

但当把这个程序变成桌面程序时,又有一个问题出现啦

         <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
                 <category android:name="android.intent.category.HOME" />
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>        

MainActivity中还有一个变量：static int []clickinfo;这个变量只在点击应用程序后面的按钮时值才会发生改变，为+号时，点击会将图标添加到主屏幕，添加完成之后立即变成-号

gv_applist决定主屏幕上加载哪些图标，但当添加完一个图标之后，立即按home键，（桌面程序被推入后台，但是不释放资源，重启桌面程序）会发现主屏幕仍然存在这个图标，但是原来应用列表里面的-号却变成了加号。

if (MainActivity.clickinfo[position] == 0) {
                ibt_operate.setImageDrawable(getResources().getDrawable(
                        R.drawable.add));
            } else {
                ibt_operate.setImageDrawable(getResources().getDrawable(
                        R.drawable.del));
            }

但应用列表里的图标仅仅依据clickinfo变量加载，所以可以断定clickinfo被初始化成全零啦。

static int []clickinfo;
public static  List<Map<String, Object>>  lv_applist = new ArrayList<Map<String,Object>>();

原来定义时，clickinfo没有使用new方法，这种变量进入后台后值是不能够保存的。

如果使用全局变量：

1、尽量减少对代码的同步计算

例如：

for(int i =0;i<list.size;i++){
 }

应替换为：

for(int i = 0, int len= list.size();i<len;i++){
   }

2、尽量采用lazy loading的策略，在需要的时候才开始创建

例如：

String str = "aaa";
    if(i==1){
     list.add(str)
    }

应替换为：

 if(i ==1){
     String str = "aaa"
      list.add(str)
    }