synchronized是Java中的關(guān)鍵字懈凹，是一種同步鎖蜀变。它修飾的對(duì)象有以下幾種：
- 修飾一個(gè)代碼塊，被修飾的代碼塊稱為同步語(yǔ)句塊介评，其作用的范圍是大括號(hào){}括起來(lái)的代碼魁亦，作用的對(duì)象是調(diào)用這個(gè)代碼塊的對(duì)象笙蒙；
- 修飾一個(gè)方法，被修飾的方法稱為同步方法，其作用的范圍是整個(gè)方法被冒，作用的對(duì)象是調(diào)用這個(gè)方法的對(duì)象；
- 修改一個(gè)靜態(tài)的方法厉萝，其作用的范圍是整個(gè)靜態(tài)方法立莉，作用的對(duì)象是這個(gè)類的所有對(duì)象；
- 修改一個(gè)類烟很，其作用的范圍是synchronized后面括號(hào)括起來(lái)的部分颈墅，作用主的對(duì)象是這個(gè)類的所有對(duì)象。
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修飾一個(gè)代碼塊
a雾袱、一個(gè)線程訪問(wèn)一個(gè)對(duì)象中的synchronized(this)同步代碼塊時(shí)恤筛，其他試圖訪問(wèn)該對(duì)象的線程將被阻塞.
/**
* @author shuliangzhao
* @Title: SyncThread
* @ProjectName design-parent
* @Description: TODO
* @date 2019/6/17 23:25
*/
public class SyncThread implements Runnable {
private static int count;
public SyncThread() {
count = 0;
}
@Override
public void run() {
synchronized (this) {
try {
for (int i = 0;i<5;i++) {
System.out.println(Thread.currentThread().getName() + ":" + (count++));
}
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
SyncThread syncThread = new SyncThread();
Thread thread = new Thread(syncThread,"Thread1");
Thread thread1 = new Thread(syncThread,"Thread2");
thread.start();
thread1.start();
}
}
運(yùn)行結(jié)果
Thread1:0
Thread1:1
Thread1:2
Thread1:3
Thread1:4
Thread2:5
Thread2:6
Thread2:7
Thread2:8
Thread2:9
當(dāng)兩個(gè)并發(fā)線程(thread1和thread2)訪問(wèn)同一個(gè)對(duì)象(syncThread)中的synchronized代碼塊時(shí)，在同一時(shí)刻只能有一個(gè)線程得到執(zhí)行芹橡，另一個(gè)線程受阻塞毒坛，必須等待當(dāng)前線程執(zhí)行完這個(gè)代碼塊以后才能執(zhí)行該代碼塊。Thread1和thread2是互斥的林说，因?yàn)樵趫?zhí)行synchronized代碼塊時(shí)會(huì)鎖定當(dāng)前的對(duì)象煎殷，只有執(zhí)行完該代碼塊才能釋放該對(duì)象鎖，下一個(gè)線程才能執(zhí)行并鎖定該對(duì)象腿箩。
把SyncThread的調(diào)用稍微改一下：
Thread thread = new Thread(new SyncThread(),"Thread1");
Thread thread1 = new Thread(new SyncThread(),"Thread2");
thread.start();
thread1.start();
運(yùn)行結(jié)果
Thread2:0
Thread1:0
Thread1:2
Thread1:3
Thread2:1
Thread2:5
Thread1:4
Thread2:6
Thread1:7
Thread2:8
這時(shí)相當(dāng)于創(chuàng)建了兩個(gè)SyncThread的對(duì)象syncThread1和syncThread2豪直，線程thread1執(zhí)行的是syncThread1對(duì)象中的synchronized代碼(run)，而線程thread2執(zhí)行的是syncThread2對(duì)象中的synchronized代碼(run)珠移；我們知道synchronized鎖定的是對(duì)象弓乙，這時(shí)會(huì)有兩把鎖分別鎖定syncThread1對(duì)象和syncThread2對(duì)象末融，而這兩把鎖是互不干擾的，不形成互斥暇韧，所以兩個(gè)線程可以同時(shí)執(zhí)行勾习。
b、當(dāng)一個(gè)線程訪問(wèn)對(duì)象的一個(gè)synchronized(this)同步代碼塊時(shí)懈玻，另一個(gè)線程仍然可以訪問(wèn)該對(duì)象中的非synchronized(this)同步代碼塊
/**
* @author shuliangzhao
* @Title: SyncThread1
* @ProjectName design-parent
* @Description: TODO
* @date 2019/6/17 23:53
*/
public class CountThread implements Runnable{
private static int count;
public CountThread() {
count = 0;
}
public void count() {
synchronized(this) {
for (int i = 0; i < 5; i ++) {
try {
System.out.println(Thread.currentThread().getName() + ":" + (count++));
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public void print() {
for (int i = 0; i < 5; i ++) {
try {
System.out.println(Thread.currentThread().getName() + ":count:" + (count));
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
@Override
public void run() {
String threadName = Thread.currentThread().getName();
if (threadName.equals("A")) {
count();
} else if (threadName.equals("B")) {
print();
}
}
public static void main(String[] args) {
CountThread countThread = new CountThread();
Thread thread1 = new Thread(countThread, "A");
Thread thread2 = new Thread(countThread, "B");
thread1.start();
thread2.start();
}
}
上面代碼中count是一個(gè)synchronized的巧婶，print是非synchronized的。從上面的結(jié)果中可以看出一個(gè)線程訪問(wèn)一個(gè)對(duì)象的synchronized代碼塊時(shí)酪刀，別的線程可以訪問(wèn)該對(duì)象的非synchronized代碼塊而不受阻塞粹舵。
c、指定要給某個(gè)對(duì)象加鎖
/**
* @author shuliangzhao
* @Title: Account
* @ProjectName design-parent
* @Description: TODO
* @date 2019/6/18 0:04
*/
public class Account {
String name;
float amount;
public Account(String name, float amount) {
this.name = name;
this.amount = amount;
}
//存錢
public void deposit(float amt) {
amount += amt;
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
//取錢
public void withdraw(float amt) {
amount -= amt;
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public float getBalance() {
return amount;
}
}
/**
* 賬戶操作類
*/
class AccountOperator implements Runnable{
private Account account;
public AccountOperator(Account account) {
this.account = account;
}
public void run() {
synchronized (account) {
account.deposit(500);
account.withdraw(500);
System.out.println(Thread.currentThread().getName() + ":" + account.getBalance());
}
}
public static void main(String[] args) {
Account account = new Account("zhang san", 10000.0f);
AccountOperator accountOperator = new AccountOperator(account);
final int THREAD_NUM = 5;
Thread threads[] = new Thread[THREAD_NUM];
for (int i = 0; i < THREAD_NUM; i ++) {
threads[i] = new Thread(accountOperator, "Thread" + i);
threads[i].start();
}
}
}
運(yùn)行結(jié)果
Thread0:10000.0
Thread4:10000.0
Thread3:10000.0
Thread2:10000.0
Thread1:10000.0
在AccountOperator 類中的run方法里骂倘，我們用synchronized 給account對(duì)象加了鎖眼滤。這時(shí)，當(dāng)一個(gè)線程訪問(wèn)account對(duì)象時(shí)历涝，其他試圖訪問(wèn)account對(duì)象的線程將會(huì)阻塞诅需，直到該線程訪問(wèn)account對(duì)象結(jié)束。也就是說(shuō)誰(shuí)拿到那個(gè)鎖誰(shuí)就可以運(yùn)行它所控制的那段代碼荧库。
當(dāng)有一個(gè)明確的對(duì)象作為鎖時(shí)堰塌，就可以用類似下面這樣的方式寫程序。
public void method3(SomeObject obj)
{
//obj 鎖定的對(duì)象
synchronized(obj)
{
// todo
}
}
當(dāng)沒(méi)有明確的對(duì)象作為鎖分衫，只是想讓一段代碼同步時(shí)场刑，可以創(chuàng)建一個(gè)特殊的對(duì)象來(lái)充當(dāng)鎖
private byte[] lock = new byte[0]; // 特殊的instance變量
public void method()
{
synchronized(lock) {
// todo 同步代碼塊
}
}
public void run() {
}
說(shuō)明：零長(zhǎng)度的byte數(shù)組對(duì)象創(chuàng)建起來(lái)將比任何對(duì)象都經(jīng)濟(jì)――查看編譯后的字節(jié)碼：生成零長(zhǎng)度的byte[]對(duì)象只需3條操作碼，而Object lock = new Object()則需要7行操作碼蚪战。
修飾一個(gè)方法
Synchronized修飾一個(gè)方法很簡(jiǎn)單牵现，就是在方法的前面加synchronized，public synchronized void method(){//todo}; synchronized修飾方法和修飾一個(gè)代碼塊類似邀桑，只是作用范圍不一樣瞎疼，修飾代碼塊是大括號(hào)括起來(lái)的范圍，而修飾方法范圍是整個(gè)函數(shù)壁畸。
public synchronized void run() {
for (int i = 0; i < 5; i ++) {
try {
System.out.println(Thread.currentThread().getName() + ":" + (count++));
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
Synchronized作用于整個(gè)方法的寫法贼急。
寫法一：
public synchronized void method()
{
// todo
}
寫法二：
public void method()
{
synchronized(this) {
// todo
}
}
在用synchronized修飾方法時(shí)要注意以下幾點(diǎn)：
- synchronized關(guān)鍵字不能繼承。
雖然可以使用synchronized來(lái)定義方法捏萍，但synchronized并不屬于方法定義的一部分太抓，因此，synchronized關(guān)鍵字不能被繼承令杈。如果在父類中的某個(gè)方法使用了synchronized關(guān)鍵字腻异，而在子類中覆蓋了這個(gè)方法，在子類中的這個(gè)方法默認(rèn)情況下并不是同步的这揣，而必須顯式地在子類的這個(gè)方法中加上synchronized關(guān)鍵字才可以悔常。當(dāng)然，還可以在子類方法中調(diào)用父類中相應(yīng)的方法给赞，這樣雖然子類中的方法不是同步的机打，但子類調(diào)用了父類的同步方法，因此片迅，子類的方法也就相當(dāng)于同步了残邀。這兩種方式的例子代碼如下：
在子類方法中加上synchronized關(guān)鍵字
class Parent {
public synchronized void method() { }
}
class Child extends Parent {
public synchronized void method() { }
}
在子類方法中調(diào)用父類的同步方法
class Parent {
public synchronized void method() { }
}
class Child extends Parent {
public void method() { super.method(); }
}
在定義接口方法時(shí)不能使用synchronized關(guān)鍵字。
構(gòu)造方法不能使用synchronized關(guān)鍵字柑蛇，但可以使用synchronized代碼塊來(lái)進(jìn)行同步
修飾一個(gè)靜態(tài)的方法
我們知道靜態(tài)方法是屬于類的而不屬于對(duì)象的芥挣。同樣的，synchronized修飾的靜態(tài)方法鎖定的是這個(gè)類的所有對(duì)象耻台，把SyncThread改造下：
/**
* @author shuliangzhao
* @Title: SyncThread
* @ProjectName design-parent
* @Description: TODO
* @date 2019/6/17 23:25
*/
public class SyncThread implements Runnable {
private static int count;
public SyncThread() {
count = 0;
}
public static void testSync() {
try {
for (int i = 0;i<5;i++) {
System.out.println(Thread.currentThread().getName() + ":" + (count++));
}
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
@Override
public synchronized void run() {
testSync();
}
public static void main(String[] args) {
SyncThread syncThread = new SyncThread();
Thread thread = new Thread(new SyncThread(),"Thread1");
Thread thread1 = new Thread(new SyncThread(),"Thread2");
thread.start();
thread1.start();
}
}
運(yùn)行結(jié)果
Thread1:0
Thread1:1
Thread1:2
Thread1:3
Thread1:4
Thread2:0
Thread2:5
Thread2:6
Thread2:7
Thread2:8
syncThread1和syncThread2是SyncThread的兩個(gè)對(duì)象空免，但在thread1和thread2并發(fā)執(zhí)行時(shí)卻保持了線程同步。這是因?yàn)閞un中調(diào)用了靜態(tài)方法method盆耽，而靜態(tài)方法是屬于類的蹋砚，所以syncThread1和syncThread2相當(dāng)于用了同一把鎖。
修飾一個(gè)類
用法
class ClassName {
public void method() {
synchronized(ClassName.class) {
// todo
}
}
}
public class SyncThread implements Runnable {
private static int count;
public SyncThread() {
count = 0;
}
public static void testSync() {
synchronized (SyncThread.class) {
try {
for (int i = 0;i<5;i++) {
System.out.println(Thread.currentThread().getName() + ":" + (count++));
}
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
@Override
public synchronized void run() {
testSync();
}
public static void main(String[] args) {
SyncThread syncThread = new SyncThread();
Thread thread = new Thread(new SyncThread(),"Thread1");
Thread thread1 = new Thread(new SyncThread(),"Thread2");
thread.start();
thread1.start();
}
}
運(yùn)行結(jié)果
Thread1:0
Thread1:1
Thread1:2
Thread1:3
Thread1:4
Thread2:5
Thread2:6
Thread2:7
Thread2:8
Thread2:9
A. 無(wú)論synchronized關(guān)鍵字加在方法上還是對(duì)象上摄杂，如果它作用的對(duì)象是非靜態(tài)的坝咐，則它取得的鎖是對(duì)象；如果synchronized作用的對(duì)象是一個(gè)靜態(tài)方法或一個(gè)類析恢，則它取得的鎖是對(duì)類墨坚，該類所有的對(duì)象同一把鎖。
B. 每個(gè)對(duì)象只有一個(gè)鎖（lock）與之相關(guān)聯(lián)映挂，誰(shuí)拿到這個(gè)鎖誰(shuí)就可以運(yùn)行它所控制的那段代碼泽篮。
C. 實(shí)現(xiàn)同步是要很大的系統(tǒng)開(kāi)銷作為代價(jià)的，甚至可能造成死鎖袖肥，所以盡量避免無(wú)謂的同步控制咪辱。
