Java:使用synchronized和Lock對象獲取對象鎖
在併發環境下，解決共享資源衝突問題時，能夠考慮使用鎖機制。
1.
對象的鎖
全部對象都自動含有單一的鎖。
JVM
負責跟蹤對象被加鎖的次數。若是一個對象被解鎖，其計數變爲
0
。在任務（線程）第一次給對象加鎖的時候，計數變爲
1
。每當這個相同的任務（線程）在此對象上得到鎖時，計數會遞增。
只有首先得到鎖的任務（線程）才能繼續獲取該對象上的多個鎖。
每當任務離開一個
synchronized
方法，計數遞減，當計數爲
0
的時候，鎖被徹底釋放，此時別的任務就能夠使用此資源。
2.synchronized
同步塊
2.1
同步到單一對象鎖
當使用同步塊時，若是方法下的同步塊都同步到一個對象上的鎖，則全部的任務（線程）只能互斥的進入這些同步塊。
Resource1.java
演示了三個線程（包括
main
線程）試圖進入某個類的三個不一樣的方法的同步塊中，雖然這些同步塊處在不一樣的方法中，但因爲是同步到同一個對象（當前對象
synchronized
(
this
)
），因此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）試圖進入某個類的三個不一樣的方法的同步塊中，這些同步塊處在不一樣的方法中，而且是同步到三個不一樣的對象（
synchronized
(
this
)
，
synchronized
(
syncObject1
)
，
synchronized
(
syncObject2
)
），因此對它們的方法中的臨界資源訪問是獨立的。
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，還能夠使用
Lock
對象來建立臨界區。
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()
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