synchronized同步代碼塊
用關(guān)鍵字synchronized聲明方法在某些情況下是有弊端的，比如A線程調(diào)用同步方法執(zhí)行一個較長時間的任務(wù)，那么B線程必須等待比較長的時間。這種情況下可以嘗試使用synchronized同步語句塊來解決問題?？匆幌吕樱?/span>
???
?public class ThreadDomain18 {
????????public void doLongTimeTask() throws Exception {
????????????for (int i = 0; i < 100; i++) {
????????????????System.out.println(
????????????????????????"nosynchronized threadName = " + Thread.currentThread().getName() + ", i = " + (i + 1));
????????????}
????????????System.out.println();
????????????synchronized (this) {
????????????????for (int i = 0; i < 100; i++) {
????????????????????System.out.println(
????????????????????????????"synchronized threadName = " + Thread.currentThread().getName() + ", i = " + (i + 1));
????????????????}
????????????}
????????}
????}
public class MyThread18 extends Thread {
????????private ThreadDomain18 td;
????????public MyThread18(ThreadDomain18 td) {
????????????this.td = td;
????????}
????????public void run() {
????????????try {
????????????????td.doLongTimeTask();
????????????} catch (Exception e) {
????????????????e.printStackTrace();
????????????}
????????}
????}
public static void main(String[] args){
?? ?ThreadDomain18 td = new ThreadDomain18();
?? ?MyThread18 mt0 = new MyThread18(td);
?? ?MyThread18 mt1 = new MyThread18(td);
?? ?mt0.start();
?? ?mt1.start();
}
運行結(jié)果，分兩部分來看：
synchronized threadName = Thread-1, i = 1synchronized threadName = Thread-1, i = 2nosynchronized threadName = Thread-0, i = 95synchronized threadName = Thread-1, i = 3nosynchronized threadName = Thread-0, i = 96synchronized threadName = Thread-1, i = 4nosynchronized threadName = Thread-0, i = 97synchronized threadName = Thread-1, i = 5nosynchronized threadName = Thread-0, i = 98synchronized threadName = Thread-1, i = 6nosynchronized threadName = Thread-0, i = 99synchronized threadName = Thread-1, i = 7nosynchronized threadName = Thread-0, i = 100...synchronized threadName = Thread-1, i = 98synchronized threadName = Thread-1, i = 99synchronized threadName = Thread-1, i = 100synchronized threadName = Thread-0, i = 1synchronized threadName = Thread-0, i = 2synchronized threadName = Thread-0, i = 3...
這個實驗可以得出以下兩個結(jié)論：
1、
當(dāng)A線程訪問對象的synchronized代碼塊的時候，B線程依然可以訪問對象方法中其余非synchronized塊的部分，第一部分的執(zhí)行結(jié)果證明了這一點
2、
當(dāng)A線程進(jìn)入對象的synchronized代碼塊的時候，B線程如果要訪問這段synchronized塊，那么訪問將會被阻塞，第二部分的執(zhí)行結(jié)果證明了這一點
所以，從執(zhí)行效率的角度考慮，有時候我們未必要把整個方法都加上synchronized，而是可以采取synchronized塊的方式，對會引起線程安全問題的那一部分代碼進(jìn)行synchronized就可以了。
兩個synchronized塊之間具有互斥性
如果線程1訪問了一個對象A方法的synchronized塊，那么線程B對同一對象B方法的synchronized塊的訪問將被阻塞，寫個例子來證明一下：
?public class ThreadDomain19 {
????????public void serviceMethodA() {
????????????synchronized (this) {
????????????????try {
????????????????????System.out.println("A begin time = " + System.currentTimeMillis());
????????????????????Thread.sleep(2000);
????????????????????System.out.println("A end time = " + System.currentTimeMillis());
????????????????} catch (InterruptedException e) {
????????????????????e.printStackTrace();
????????????????}
????????????}
????????}
????????public void serviceMethodB() {
????????????synchronized (this) {
????????????????System.out.println("B begin time = " + System.currentTimeMillis());
????????????????System.out.println("B end time = " + System.currentTimeMillis());
????????????}
????????}
????}
寫兩個線程分別調(diào)用這兩個方法：
public class MyThread19_0 extends Thread{
?? ?private ThreadDomain19 td;
?? ?public MyThread19_0(ThreadDomain19 td){
?? ??? ?this.td = td;
?? ?}
?? ?public void run(){
?? ??? ?td.serviceMethodA();
?? ?}
}
public class MyThread19_1 extends Thread{
?? ?private ThreadDomain19 td;
?? ?public MyThread19_1(ThreadDomain19 td){
?? ??? ?this.td = td;
?? ?}
?? ?public void run(){
?? ??? ?td.serviceMethodB();
?? ?}
}
寫個main函數(shù)：
public static void main(String[] args){
?? ?ThreadDomain19 td = new ThreadDomain19();
?? ?MyThread19_0 mt0 = new MyThread19_0(td);
?? ?MyThread19_1 mt1 = new MyThread19_1(td);
?? ?mt0.start();
?? ?mt1.start();
}
看一下運行結(jié)果：
A begin time = 1443843271982A end time = 1443843273983B begin time = 1443843273983B end time = 1443843273983
看到對于serviceMethodB()方法synchronized塊的訪問必須等到對于serviceMethodA()方法synchronized塊的訪問結(jié)束之后。那其實這個例子，我們也可以得出一個結(jié)論：
synchronized塊獲得的是一個對象鎖，換句話說，synchronized塊鎖定的是整個對象
。
synchronized塊和synchronized方法
既然上面得到了一個結(jié)論
synchronized塊獲得的是對象鎖
，那么如果線程1訪問了一個對象方法A的synchronized塊，線程2對于同一對象同步方法B的訪問應(yīng)該是會被阻塞的，因為線程2訪問同一對象的同步方法B的時候?qū)L試去獲取這個對象的對象鎖，但這個鎖卻在線程1這里。寫一個例子證明一下這個結(jié)論：
???
?public class ThreadDomain20 {
????????public synchronized void otherMethod() {
????????????System.out.println("----------run--otherMethod");
????????}
????????public void doLongTask() {
????????????synchronized (this) {
????????????????for (int i = 0; i < 1000; i++) {
????????????????????System.out.println(
????????????????????????????"synchronized threadName = " + Thread.currentThread().getName() + ", i = " + (i + 1));
????????????????????try {
????????????????????????Thread.sleep(5);
????????????????????} catch (InterruptedException e) {
????????????????????????e.printStackTrace();
????????????????????}
????????????????}
????????????}
????????}
????}
寫兩個線程分別調(diào)用這兩個方法：
public class MyThread20_0 extends Thread{
?? ?private ThreadDomain20 td;
?? ?public MyThread20_0(ThreadDomain20 td){
?? ??? ?this.td = td;
?? ?}
?? ?public void run(){
?? ??? ?td.doLongTask();
?? ?}
}
public class MyThread20_1 extends Thread{
?? ?private ThreadDomain20 td;
?? ?public MyThread20_1(ThreadDomain20 td){
?? ??? ?this.td = td;
?? ?}
?? ?public void run(){
?? ??? ?td.otherMethod();
?? ?}
}
寫個main函數(shù)調(diào)用一下，這里"mt0.start()"后sleep(100)以下是為了確保mt0線程先啟動：
public static void main(String[] args) throws Exception{
?? ?ThreadDomain20 td = new ThreadDomain20();
?? ?MyThread20_0 mt0 = new MyThread20_0(td);
?? ?MyThread20_1 mt1 = new MyThread20_1(td);
?? ?mt0.start();
?? ?Thread.sleep(100);
?? ?mt1.start();
}
看一下運行結(jié)果：
...synchronized threadName = Thread-0, i = 995synchronized threadName = Thread-0, i = 996synchronized threadName = Thread-0, i = 997synchronized threadName = Thread-0, i = 998synchronized threadName = Thread-0, i = 999synchronized threadName = Thread-0, i = 1000----------run--otherMethod
證明了我們的結(jié)論。為了進(jìn)一步完善這個結(jié)論，把"otherMethod()"方法的synchronized去掉再看一下運行結(jié)果：
...synchronized threadName = Thread-0, i = 16synchronized threadName = Thread-0, i = 17synchronized threadName = Thread-0, i = 18synchronized threadName = Thread-0, i = 19synchronized threadName = Thread-0, i = 20----------run--otherMethodsynchronized threadName = Thread-0, i = 21synchronized threadName = Thread-0, i = 22synchronized threadName = Thread-0, i = 23...
"otherMethod()"方法和"doLongTask()"方法中的synchronized塊異步執(zhí)行了
將任意對象作為對象監(jiān)視器
總結(jié)一下前面的內(nèi)容：
1、synchronized同步方法
（1）對其他synchronized同步方法或synchronized(this)同步代碼塊呈阻塞狀態(tài)
（2）同一時間只有一個線程可以執(zhí)行synchronized同步方法中的代碼
2、synchronized同步代碼塊
（1）對其他synchronized同步方法或synchronized(this)同步代碼塊呈阻塞狀態(tài)
（2）同一時間只有一個線程可以執(zhí)行synchronized(this)同步代碼塊中的代碼
前面都使用synchronized(this)的格式來同步代碼塊，其實
Java還支持對"任意對象"作為對象監(jiān)視器來實現(xiàn)同步的功能
。這個"任意對象"大多數(shù)是
實例變量
及
方法的參數(shù)
，使用格式為synchronized(非this對象)?？匆幌聦⑷我鈱ο笞鳛閷ο蟊O(jiān)視器的使用例子：
public class ThreadDomain21 {
????????private String userNameParam;
????????private String passwordParam;
????????private String anyString = new String();
????????public void setUserNamePassword(String userName, String password) {
????????????try {
????????????????synchronized (anyString) {
????????????????????System.out.println("線程名稱為：" + Thread.currentThread().getName() + "在 " + System.currentTimeMillis()?+ " 進(jìn)入同步代碼塊");
????????????????????userNameParam = userName;
????????????????????Thread.sleep(3000);
????????????????????passwordParam = password;
????????????????????System.out.println("線程名稱為：" + Thread.currentThread().getName() + "在 " + System.currentTimeMillis()?+ " 離開同步代碼塊");
????????????????}
????????????} catch (InterruptedException e) {
????????????????e.printStackTrace();
????????????}
????????}
????}
寫兩個線程分別調(diào)用一下：
public class MyThread21_0 extends Thread{
?? ?private ThreadDomain21 td;
?? ?public MyThread21_0(ThreadDomain21 td){
?? ??? ?this.td = td;
?? ?}
?? ?public void run(){
?? ??? ?td.setUserNamePassword("A", "AA");
?? ?}
}
public class MyThread21_1 extends Thread{
?? ?private ThreadDomain21 td;
?? ?public MyThread21_1(ThreadDomain21 td){
?? ??? ?this.td = td;
?? ?}
?? ?public void run(){
?? ??? ?td.setUserNamePassword("B", "B");
?? ?}
}
寫一個main函數(shù)調(diào)用一下：
public static void main(String[] args){
?? ?ThreadDomain21 td = new ThreadDomain21();
?? ?MyThread21_0 mt0 = new MyThread21_0(td);
?? ?MyThread21_1 mt1 = new MyThread21_1(td);
?? ?mt0.start();
?? ?mt1.start();
}
看一下運行結(jié)果：
線程名稱為：Thread-0在 1443855101706 進(jìn)入同步代碼塊線程名稱為：Thread-0在 1443855104708 離開同步代碼塊線程名稱為：Thread-1在 1443855104708 進(jìn)入同步代碼塊線程名稱為：Thread-1在 1443855107708 離開同步代碼塊
這個例子證明了：
多個線程持有"對象監(jiān)視器"為同一個對象的前提下，同一時間只能有一個線程可以執(zhí)行synchronized(非this對象x)代碼塊中的代碼
。
鎖非this對象具有一定的優(yōu)點：如果在一個類中有很多synchronized方法，這時雖然能實現(xiàn)同步，但會受到阻塞，從而影響效率。但如果同步代碼塊鎖的是非this對象，則synchronized(非this對象x)代碼塊中的程序與同步方法是異步的，不與其他鎖this同步方法爭搶this鎖，大大提高了運行效率。
注意一下"private String anyString = new String();"這句話，現(xiàn)在它是一個全局對象，因此監(jiān)視的是同一個對象。如果移到try里面，那么對象的監(jiān)視器就不是同一個了，調(diào)用的時候自然是異步調(diào)用，可以自己試一下。
最后提一點，synchronized(非this對象x)，這個對象如果是實例變量的話，指的是對象的引用，
只要對象的引用不變，即使改變了對象的屬性，運行結(jié)果依然是同步的
。
細(xì)化synchronized(非this對象x)的三個結(jié)論
synchronized(非this對象x)格式的寫法是將x對象本身作為對象監(jiān)視器，有三個結(jié)論得出：
1、當(dāng)多個線程同時執(zhí)行synchronized(x){}同步代碼塊時呈同步效果
2、當(dāng)其他線程執(zhí)行x對象中的synchronized同步方法時呈同步效果
3、當(dāng)其他線程執(zhí)行x對象方法中的synchronized(this)代碼塊時也呈同步效果
第一點很明顯，第二點和第三點意思類似，無非一個是同步方法，一個是同步代碼塊罷了，舉個例子驗證一下第二點：
?
public class MyObject {
????????public synchronized void speedPrintString() {
????????????System.out.println("speedPrintString__getLock time = " + System.currentTimeMillis() + ", run ThreadName = "?+ Thread.currentThread().getName());
????????????System.out.println("----------");
????????????System.out.println("speedPrintString__releaseLock time = " + System.currentTimeMillis()+ ", run ThreadName = " + Thread.currentThread().getName());
????????}
????}
ThreadDomain24中持有MyObject的引用：
public class ThreadDomain24 {
????????public void testMethod1(MyObject mo) {
????????????try {
????????????????synchronized (mo) {
????????????????????System.out.println("testMethod1__getLock time = " + System.currentTimeMillis()?+ ", run ThreadName = " + Thread.currentThread().getName());
????????????????????Thread.sleep(5000);
????????????????????System.out.println("testMethod1__releaseLock time = " + System.currentTimeMillis()?+ ", run ThreadName = " + Thread.currentThread().getName());
????????????????}
????????????} catch (InterruptedException e) {
????????????????e.printStackTrace();
????????????}?
????????}
????}
寫兩個線程分別調(diào)用"
speedPrintString()
"方法和"
testMethod1(MyObject mo)
"方法：
public class MyThread24_0 extends Thread{
?? ?private ThreadDomain24 td;
?? ?private MyObject mo;
?? ?public MyThread24_0(ThreadDomain24 td, MyObject mo){
?? ??? ?this.td = td;
?? ??? ?this.mo = mo;
?? ?}
?? ?public void run() {
?? ??? ?td.testMethod1(mo);
?? ?}
}
public class MyThread24_1 extends Thread{
?? ?private MyObject mo;
?? ?public MyThread24_1(MyObject mo){
?? ??? ?this.mo = mo;
?? ?}
?? ?public void run(){
?? ??? ?mo.speedPrintString();
?? ?}
}
寫一個main函數(shù)啟動這兩個線程：
public static void main(String[] args){
?? ?ThreadDomain24 td = new ThreadDomain24();
?? ?MyObject mo = new MyObject();
?? ?MyThread24_0 mt0 = new MyThread24_0(td, mo);
?? ?MyThread24_1 mt1 = new MyThread24_1(mo);
?? ?mt0.start();
?? ?mt1.start();
}
看一下運行結(jié)果：
testMethod1__getLock time = 1443855939811, run ThreadName = Thread-0testMethod1__releaseLock time = 1443855944812, run ThreadName = Thread-0speedPrintString__getLock time = 1443855944812, run ThreadName = Thread-1----------speedPrintString__releaseLock time = 1443855944812, run ThreadName = Thread-1
看到"speedPrintString()"方法必須等待"testMethod1(MyObject mo)"方法執(zhí)行完畢才可以執(zhí)行，沒有辦法異步執(zhí)行，證明了第二點的結(jié)論。第三點的驗證方法類似，就不寫代碼證明了。
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