synchronized關(guān)鍵字的使用

• 在日常開(kāi)發(fā)中我們經(jīng)?？吹絪ynchronized關(guān)鍵字,常見(jiàn)的幾種應(yīng)用是synchronized(this)、synchronized(class)與synchronized(Object)，和synchronize方法，靜態(tài)同步synchronized方法，這幾種方式究竟有什么區(qū)別呢，當(dāng)初學(xué)習(xí)java同步鎖那塊的時(shí)候好像就沒(méi)怎么注意過(guò)，現(xiàn)在來(lái)通過(guò)實(shí)例比較下：

• 對(duì)于synchronized(this):

    //核心業(yè)務(wù)代碼
    public class HotProductService {
        public void serviceMethodA(){//業(yè)務(wù)A
            try {
                synchronized (this) {
                    Log.d("HotProductService","A begin time="+System.currentTimeMillis());
                    Thread.sleep(2000);
                    Log.d("HotProductService","A end   time="+System.currentTimeMillis());
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        public void serviceMethodB(){
            synchronized (this) {
                Log.d("HotProductService","B begin time="+System.currentTimeMillis());
                Log.d("HotProductService","B end   time="+System.currentTimeMillis());
            }
        }
    }
  
    //線程1
    public class ThreadA extends Thread{
        private HotProductService objectService;
        public ThreadA(HotProductService objectService){
            super();
            this.objectService=objectService;
        }
        @Override
        public void run() {
            super.run();//調(diào)用業(yè)務(wù)a
            objectService.serviceMethodA();
        }
    }
    //線程2
    public class ThreadB extends Thread {
        private HotProductService objectService;
        public ThreadB(HotProductService objectService){
            super();
            this.objectService=objectService;
        }
        @Override
        public void run() {
            super.run()；//調(diào)用業(yè)務(wù)b
            objectService.serviceMethodB();
        }
    }
    //測(cè)試代碼
    private void test1() {
            HotProductService service=new HotProductService();
            ThreadA a=new ThreadA(service);
            a.start();
            ThreadB b=new ThreadB(service);
            b.start();
        }
  

運(yùn)行結(jié)果:

    02-24 10:05:12.135 6434-7477/com.example.refreshdemo D/HotProductService: A begin time=1519437912141
    02-24 10:05:14.135 6434-7477/com.example.refreshdemo D/HotProductService: A end   time=1519437914142
    02-24 10:05:14.135 6434-7478/com.example.refreshdemo D/HotProductService: B begin time=1519437914145
    02-24 10:05:14.135 6434-7478/com.example.refreshdemo D/HotProductService: B end   time=1519437914146

上面的結(jié)果我們能看到即使A業(yè)務(wù)耗時(shí)較長(zhǎng)，B業(yè)務(wù)也并不會(huì)獲得執(zhí)行的機(jī)會(huì)，如果我們將核心業(yè)務(wù)代碼中B業(yè)務(wù)或者A業(yè)務(wù)的同步鎖去掉，其他照舊會(huì)是怎么樣

    ...
    比如去掉B業(yè)務(wù)的同步鎖
    public void serviceMethodB(){
    //synchronized (this) 
        {
        Log.d("HotProductService","B begin time="+System.currentTimeMillis());
        Log.d("HotProductService","B end   time="+System.currentTimeMillis());
        }
    }
    ...

運(yùn)行結(jié)果:

    02-24 02:27:19.248 3562-3629/com.example.refreshdemo D/HotProductService: A begin time=1519439239248
    02-24 02:27:19.248 3562-3630/com.example.refreshdemo D/HotProductService: B begin time=1519439239248
    02-24 02:27:19.248 3562-3630/com.example.refreshdemo D/HotProductService: B end   time=1519439239248
    02-24 02:27:21.248 3562-3629/com.example.refreshdemo D/HotProductService: A end   time=1519439241248

結(jié)論：synchronized (this)使用的對(duì)象監(jiān)視器是一個(gè),即是該對(duì)象自身，當(dāng)一個(gè)線程訪問(wèn)HotProductService的一個(gè)synchronized (this)同步代碼塊時(shí)，其它線程對(duì)同一個(gè)HotProductService中其它的synchronized (this)同步代碼塊的訪問(wèn)將是堵塞，實(shí)現(xiàn)了代碼順序的同步執(zhí)行。

• 對(duì)于synchronized修飾的方法:
  上面的結(jié)論對(duì)于synchronized修飾的方法是否同樣有效呢，通過(guò)實(shí)例看下:

    //核心業(yè)務(wù)代碼
    public class HotProductService {
        //業(yè)務(wù)A
        public synchronized void objectMethodA(){
            Log.d("HotProductService","run----objectMethodA2");
        }
        //業(yè)務(wù)B
        public void objectMethodB(){
            synchronized (this) {
                try {
                    for (int i = 1; i <= 10; i++) {
                        Log.d("HotProductService","synchronized thread name:"+Thread.currentThread().getName()+"-->i="+i);
                        Thread.sleep(1000);
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
    
    //A線程
    public class ThreadA2 extends Thread{
        private HotProductService objectService;
        public ThreadA2(HotProductService objectService){
            super();
            this.objectService=objectService;
        }
        @Override
        public void run() {
            super.run();
            objectService.objectMethodA();
        }
    }
  
    //B線程
    public class ThreadB2 extends Thread {
        private HotProductService objectService;
        public ThreadB2(HotProductService objectService){
            super();
            this.objectService=objectService;
        }
        @Override
        public void run() {
            super.run();
            objectService.objectMethodB();
        }
    }
    
    private void test2() {
            HotProductService service=new HotProductService();
            ThreadA2 a=new ThreadA2(service);
            a.start();
            ThreadB2 b=new ThreadB2(service);
            b.start();
        }
  

運(yùn)行結(jié)果：

    02-24 02:48:32.212 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=1
    02-24 02:48:33.213 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=2
    02-24 02:48:34.214 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=3
    02-24 02:48:35.215 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=4
    02-24 02:48:36.216 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=5
    02-24 02:48:37.217 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=6
    02-24 02:48:38.218 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=7
    02-24 02:48:39.219 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=8
    02-24 02:48:40.220 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=9
    02-24 02:48:41.221 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=10
    02-24 02:48:42.223 3562-3764/com.example.refreshdemo D/HotProductService: run----objectMethodA2

同樣驗(yàn)證了只能同時(shí)被一個(gè)線程所訪問(wèn)synchronized代碼塊。其他線程遇到synchronized代碼塊將會(huì)堵塞。

• 那么synchronized(Object)呢

    //核心業(yè)務(wù)代碼
        public class HotProductService {
            private String uname;
            private String pwd;
            String lock=new String();
            public void setUserNamePassWord(String userName,String passWord){
                try {
                    synchronized (lock) {
                        Log.d("HotProductService","thread name=" + Thread.currentThread().getName()
                                + " 進(jìn)入代碼快:" + System.currentTimeMillis());
                        uname=userName;
                        Thread.sleep(3000);
                        pwd=passWord;
                        Log.d("HotProductService","thread name="+Thread.currentThread().getName()
                                +" 進(jìn)入代碼快:"+System.currentTimeMillis()+"入?yún)name:"+uname+"入?yún)wd:"+pwd);
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
  
    //A線程
    public class ThreadA3 extends Thread{
        private HotProductService objectService;
        public ThreadA3(HotProductService objectService){
            super();
            this.objectService=objectService;
        }
        @Override
        public void run() {
            super.run();
            objectService.setUserNamePassWord("a","aa");
        }
    }
  
    //B線程
    public class ThreadB3 extends Thread {
        private HotProductService objectService;
        public ThreadB3(HotProductService objectService){
            super();
            this.objectService=objectService;
        }
        @Override
        public void run() {
            super.run();
            objectService.setUserNamePassWord("b","bb");
        }
    }
  
    HotProductService service=new HotProductService();
    ThreadA2 a=new ThreadA2(service);
    a.start();
    ThreadB2 b=new ThreadB2(service);
    b.start();
  

運(yùn)行結(jié)果：

    02-24 02:57:41.380 3975-4016/? D/HotProductService: thread name=A 進(jìn)入代碼快:1519441061380
    02-24 02:57:44.382 3975-4016/com.example.refreshdemo D/HotProductService: thread name=A 進(jìn)入代碼快:1519441064382入?yún)name:a入?yún)wd:aa
    02-24 02:57:44.382 3975-4017/com.example.refreshdemo D/HotProductService: thread name=B 進(jìn)入代碼快:1519441064382
    02-24 02:57:47.383 3975-4017/com.example.refreshdemo D/HotProductService: thread name=B 進(jìn)入代碼快:1519441067383入?yún)name:b入?yún)wd:bb

如果我們將

    //核心業(yè)務(wù)代碼
        public class HotProductService {
            private String uname;
            private String pwd;
           //String lock=new String();移入方法會(huì)得到什么樣的結(jié)果呢
            public void setUserNamePassWord(String userName,String passWord){
                try {
                    String lock=new String();
                    synchronized (lock) {
                        Log.d("HotProductService","thread name=" + Thread.currentThread().getName()
                                + " 進(jìn)入代碼快:" + System.currentTimeMillis());
                        uname=userName;
                        Thread.sleep(3000);
                        pwd=passWord;
                        Log.d("HotProductService","thread name="+Thread.currentThread().getName()
                                +" 進(jìn)入代碼快:"+System.currentTimeMillis()+"入?yún)name:"+uname+"入?yún)wd:"+pwd);
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }

其他代碼照舊得到
運(yùn)行結(jié)果：

    02-24 03:04:16.757 4148-4187/? D/HotProductService: thread name=A 進(jìn)入代碼快:1519441456757
    02-24 03:04:16.758 4148-4188/? D/HotProductService: thread name=B 進(jìn)入代碼快:1519441456758
    02-24 03:04:19.758 4148-4187/com.example.refreshdemo D/HotProductService: thread name=A 進(jìn)入代碼快:1519441459758入?yún)name:b入?yún)wd:aa
    02-24 03:04:19.759 4148-4188/com.example.refreshdemo D/HotProductService: thread name=B 進(jìn)入代碼快:1519441459759入?yún)name:b入?yún)wd:bb

是不是很奇怪出錯(cuò)了？為什么呢 移入方法內(nèi)部就不對(duì)了，比較前后改變，發(fā)現(xiàn)修改后每次調(diào)用方法時(shí)（哪個(gè)線程調(diào)用方法時(shí)）都會(huì)重新new一個(gè)對(duì)象作為對(duì)象鎖，但是修改前不管哪次進(jìn)入方法時(shí)（哪個(gè)線程調(diào)用方法時(shí)）用的都是同一個(gè)對(duì)象，所以我們得到結(jié)論:

使用自定義任意對(duì)象進(jìn)行同步鎖 不同線程必須為同一對(duì)象，否則仍舊是異步運(yùn)行的

• synchronized(.class)與靜態(tài)同步synchronized方法

可以直接給出的結(jié)論是這兩種方式的同步是一樣的，使用的同步鎖都是對(duì)應(yīng)的類(lèi)作為對(duì)象鎖，在jvm中類(lèi)是唯一的，那也就是說(shuō)對(duì)任何對(duì)象都是同步的，因?yàn)榫哂形ㄒ坏膶?duì)象鎖。下面來(lái)驗(yàn)證下：

    //核心業(yè)務(wù)代碼
    public class HotProductService {
        //業(yè)務(wù)A
        public  void methodA(){
            try {
                synchronized (HotProductService.class){
                    Log.d("HotProductService","static methodA begin Name:" + Thread.currentThread().getName() + " times:" + System.currentTimeMillis());
                    Thread.sleep(2000);
                    Log.d("HotProductService","static methodA end   Name:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis());
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        //業(yè)務(wù)B
        public  void methodB(){
            synchronized (HotProductService.class){
                Log.d("HotProductService","static methodB begin Name:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis());
                Log.d("HotProductService","static methodB end   Name:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis());
            }
        }
    }

    //A線程
    public class ThreadA5 extends Thread{
    
        private  HotProductService mHotProductService;
    
        public ThreadA5(HotProductService hotProductService){
            mHotProductService = hotProductService;
        }
        @Override
        public void run() {
            super.run();
            mHotProductService.methodA();
        }
    }
    
    //B線程
    public class ThreadB5 extends Thread {
    
        private  HotProductService mHotProductService;
    
        public ThreadB5(HotProductService hotProductService){
            mHotProductService = hotProductService;
        }
        @Override
        public void run() {
            super.run();
            mHotProductService.methodB();
        }
    }

    private void test5() {
        HotProductService service= new HotProductService();
        ThreadA5 a=new ThreadA5(service);
        a.start();
        ThreadB5 b=new ThreadB5(service);
        b.start();
    }

運(yùn)行結(jié)果：

    02-24 03:24:11.250 4148-4282/com.example.refreshdemo D/HotProductService: static methodA begin Name:Thread-174 times:1519442651250
    02-24 03:24:13.252 4148-4282/com.example.refreshdemo D/HotProductService: static methodA end   Name:Thread-174 times:1519442653252
    02-24 03:24:13.252 4148-4283/com.example.refreshdemo D/HotProductService: static methodB begin Name:Thread-175 times:1519442653252
    02-24 03:24:13.252 4148-4283/com.example.refreshdemo D/HotProductService: static methodB end   Name:Thread-175 times:1519442653252

使用.class作為對(duì)象鎖保證了不能線程的同步。
引用網(wǎng)上常見(jiàn)的說(shuō)法就是:

synchronize修飾的方法和 synchronize(this) 都是鎖住自己本身的對(duì)象 而synchronize(class) synchronize(object) 都是鎖別的對(duì)象