在并發(fā)環(huán)境下，解決共享資源沖突問題時，可以考慮使用鎖機(jī)制。
1.對象的鎖
所有對象都自動含有單一的鎖。
JVM負(fù)責(zé)跟蹤對象被加鎖的次數(shù)。如果一個對象被解鎖，其計數(shù)變?yōu)?。在任務(wù)（線程）第一次給對象加鎖的時候，計數(shù)變?yōu)?。每當(dāng)這個相同的任務(wù)（線程）在此對象上獲得鎖時，計數(shù)會遞增。
只有首先獲得鎖的任務(wù)（線程）才能繼續(xù)獲取該對象上的多個鎖。
每當(dāng)任務(wù)離開一個synchronized方法，計數(shù)遞減，當(dāng)計數(shù)為0的時候，鎖被完全釋放，此時別的任務(wù)就可以使用此資源。
2.synchronized同步塊
2.1同步到單一對象鎖
當(dāng)使用同步塊時，如果方法下的同步塊都同步到一個對象上的鎖，則所有的任務(wù)（線程）只能互斥的進(jìn)入這些同步塊。
Resource1.java演示了三個線程（包括main線程）試圖進(jìn)入某個類的三個不同的方法的同步塊中，雖然這些同步塊處在不同的方法中，但由于是同步到同一個對象（當(dāng)前對象 synchronized (this)），所以對它們的方法依然是互斥的。
Resource1.java
package com.zj.lock;
import java.util.concurrent.TimeUnit;
public class Resource1 {
public void f() {
// other operations should not be locked...
System.out.println(Thread.currentThread().getName()
+ ":not synchronized in f()");
synchronized (this) {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()
+ ":synchronized in f()");
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public void g() {
// other operations should not be locked...
System.out.println(Thread.currentThread().getName()
+ ":not synchronized in g()");
synchronized (this) {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()
+ ":synchronized in g()");
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public void h() {
// other operations should not be locked...
System.out.println(Thread.currentThread().getName()
+ ":not synchronized in h()");
synchronized (this) {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()
+ ":synchronized in h()");
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public static void main(String[] args) {
final Resource1 rs = new Resource1();
new Thread() {
public void run() {
rs.f();
}
}.start();
new Thread() {
public void run() {
rs.g();
}
}.start();
rs.h();
}
}
結(jié)果：
Thread-0:not synchronized in f()
Thread-0:synchronized in f()
main:not synchronized in h()
Thread-1:not synchronized in g()
Thread-0:synchronized in f()
Thread-0:synchronized in f()
Thread-0:synchronized in f()
Thread-0:synchronized in f()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
main:synchronized in h()
main:synchronized in h()
main:synchronized in h()
main:synchronized in h()
main:synchronized in h()
2.2 同步到多個對象鎖
Resource1.java演示了三個線程（包括main線程）試圖進(jìn)入某個類的三個不同的方法的同步塊中，這些同步塊處在不同的方法中，并且是同步到三個不同的對象（synchronized (this)，synchronized (syncObject1)，synchronized (syncObject2)），所以對它們的方法中的臨界資源訪問是獨(dú)立的。
Resource2.java
package com.zj.lock;
import java.util.concurrent.TimeUnit;
public class Resource2 {
private Object syncObject1 = new Object();
private Object syncObject2 = new Object();
public void f() {
// other operations should not be locked...
System.out.println(Thread.currentThread().getName()
+ ":not synchronized in f()");
synchronized (this) {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()
+ ":synchronized in f()");
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public void g() {
// other operations should not be locked...
System.out.println(Thread.currentThread().getName()
+ ":not synchronized in g()");
synchronized (syncObject1) {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()
+ ":synchronized in g()");
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public void h() {
// other operations should not be locked...
System.out.println(Thread.currentThread().getName()
+ ":not synchronized in h()");
synchronized (syncObject2) {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()
+ ":synchronized in h()");
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public static void main(String[] args) {
final Resource2 rs = new Resource2();
new Thread() {
public void run() {
rs.f();
}
}.start();
new Thread() {
public void run() {
rs.g();
}
}.start();
rs.h();
}
}
結(jié)果：
Thread-0:not synchronized in f()
Thread-0:synchronized in f()
main:not synchronized in h()
main:synchronized in h()
Thread-1:not synchronized in g()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
3.Lock對象鎖
除了使用synchronized外，還可以使用Lock對象來創(chuàng)建臨界區(qū)。Resource3.java的演示效果同Resource1.java；Resource4.java的演示效果同Resource2.java。
Resource3.java
package com.zj.lock;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Resource3 {
private Lock lock = new ReentrantLock();
public void f() {
// other operations should not be locked...
System.out.println(Thread.currentThread().getName()
+ ":not synchronized in f()");
lock.lock();
try {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()
+ ":synchronized in f()");
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} finally {
lock.unlock();
}
}
public void g() {
// other operations should not be locked...
System.out.println(Thread.currentThread().getName()
+ ":not synchronized in g()");
lock.lock();
try {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()
+ ":synchronized in g()");
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} finally {
lock.unlock();
}
}
public void h() {
// other operations should not be locked...
System.out.println(Thread.currentThread().getName()
+ ":not synchronized in h()");
lock.lock();
try {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()
+ ":synchronized in h()");
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} finally {
lock.unlock();
}
}
public static void main(String[] args) {
final Resource3 rs = new Resource3();
new Thread() {
public void run() {
rs.f();
}
}.start();
new Thread() {
public void run() {
rs.g();
}
}.start();
rs.h();
}
}
結(jié)果：
Thread-0:not synchronized in f()
Thread-0:synchronized in f()
main:not synchronized in h()
Thread-1:not synchronized in g()
Thread-0:synchronized in f()
Thread-0:synchronized in f()
Thread-0:synchronized in f()
Thread-0:synchronized in f()
main:synchronized in h()
main:synchronized in h()
main:synchronized in h()
main:synchronized in h()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Resource4.java
package com.zj.lock;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Resource4 {
private Lock lock1 = new ReentrantLock();
private Lock lock2 = new ReentrantLock();
private Lock lock3 = new ReentrantLock();
public void f() {
// other operations should not be locked...
System.out.println(Thread.currentThread().getName()
+ ":not synchronized in f()");
lock1.lock();
try {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()
+ ":synchronized in f()");
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} finally {
lock1.unlock();
}
}
public void g() {
// other operations should not be locked...
System.out.println(Thread.currentThread().getName()
+ ":not synchronized in g()");
lock2.lock();
try {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()
+ ":synchronized in g()");
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} finally {
lock2.unlock();
}
}
public void h() {
// other operations should not be locked...
System.out.println(Thread.currentThread().getName()
+ ":not synchronized in h()");
lock3.lock();
try {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()
+ ":synchronized in h()");
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} finally {
lock3.unlock();
}
}
public static void main(String[] args) {
final Resource4 rs = new Resource4();
new Thread() {
public void run() {
rs.f();
}
}.start();
new Thread() {
public void run() {
rs.g();
}
}.start();
rs.h();
}
}
結(jié)果：
Thread-0:not synchronized in f()
Thread-0:synchronized in f()
main:not synchronized in h()
main:synchronized in h()
Thread-1:not synchronized in g()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
標(biāo)簽：
本站文章除注明轉(zhuǎn)載外，均為本站原創(chuàng)或翻譯。歡迎任何形式的轉(zhuǎn)載，但請務(wù)必注明出處、不得修改原文相關(guān)鏈接，如果存在內(nèi)容上的異議請郵件反饋至chenjj@fc6vip.cn
文章轉(zhuǎn)載自：網(wǎng)絡(luò)轉(zhuǎn)載
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