要求
啟動N個線程, 這N個線程要不間斷按順序打印數字1-N. 將問題簡化為3個線程無限循環打印1到3
方法一: 使用synchronized
三個線程無序競爭同步鎖, 如果遇上的是自己的數字, 就打印. 這種方式會浪費大量的循環
public class TestSequential1 {
private volatile int pos = 1;
private volatile int count = 0;
public void one(int i) {
synchronized (this) {
if (pos == i) {
System.out.println("T-" + i + " " + count);
pos = i % 3 + 1;
count = 0;
} else {
count++;
}
}
}
public static void main(String[] args) {
TestSequential1 demo = new TestSequential1();
for (int i = 1; i <=3; i++) {
int j = i;
new Thread(()->{
while(true) {
demo.one(j);
}
}).start();
}
}
}
輸出
T-1 0 T-2 5793 T-3 5285 T-1 2616 T-2 33 T-3 28 T-1 22 T-2 44 T-3 6 T-1 881 T-2 118358 T-3 247380 T-1 30803 T-2 29627 T-3 52044 ...
方法二: 使用synchronized配合wait()和notifyAll()
競爭同步鎖時使用wait()和notifyAll(), 可以避免浪費循環
public class TestSequential4 {
private volatile int pos = 1;
private volatile int count = 0;
private final Object obj = new Object();
public void one(int i) {
System.out.println(i + " try");
synchronized (obj) {
System.out.println(i + " in");
try {
while (pos != i) {
count++;
System.out.println(i + " wait");
obj.wait();
}
System.out.println("T-" + i + " " + count);
pos = i % 3 + 1;
count = 0;
obj.notifyAll();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
TestSequential4 demo = new TestSequential4();
for (int i = 3; i >=1; i--) {
int j = i;
new Thread(()->{
while(true) {
demo.one(j);
}
}).start();
}
}
}
輸出
3 try 3 in 3 wait 2 try 2 in 2 wait 1 try 1 in T-1 2 1 try 1 in 1 wait T-2 1 2 try 2 in 2 wait T-3 1 3 try 3 in 3 wait 2 wait T-1 2 1 try 1 in 1 wait T-2 1 2 try 2 in 2 wait T-3 1 3 try 3 in 3 wait 2 wait T-1 2 1 try 1 in 1 wait T-2 1 2 try 2 in 2 wait T-3 1 3 try 3 in 3 wait 2 wait T-1 2 1 try 1 in 1 wait T-2 1 2 try 2 in 2 wait T-3 1 3 try 3 in 3 wait 2 wait T-1 2 1 try 1 in 1 wait T-2 1 2 try 2 in 2 wait T-3 1 3 try 3 in 3 wait 2 wait T-1 2 1 try 1 in 1 wait T-2 1 2 try 2 in 2 wait T-3 1 3 try 3 in 3 wait 2 wait T-1 2 1 try 1 in 1 wait T-2 1 2 try 2 in 2 wait T-3 1 3 try 3 in 3 wait 2 wait T-1 2 1 try 1 in 1 wait T-2 1 2 try 2 in 2 wait T-3 1 3 try 3 in 3 wait 2 wait T-1 2 1 try 1 in 1 wait T-2 1 2 try 2 in 2 wait T-3 1 3 try 3 in 3 wait 2 wait T-1 2 ...
.
方法三: 使用可重入鎖
用Lock做, 非公平鎖, 三個線程競爭, 如果遇上的是自己的數字, 就打印. 這種方式也會浪費大量的循環
public class TestSequential2 {
private final Lock lock = new ReentrantLock();
private volatile int pos = 1;
private volatile int count = 0;
public void one(int i) {
lock.lock();
if (pos == i) {
System.out.println("T-" + i + " " + count);
pos = i % 3 + 1;
count = 0;
} else {
count++;
}
lock.unlock();
}
public static void main(String[] args) {
TestSequential2 demo = new TestSequential2();
for (int i = 1; i <=3; i++) {
int j = i;
new Thread(()->{
while(true) {
demo.one(j);
}
}).start();
}
}
}
輸出
T-1 0 T-2 0 T-3 323 T-1 54 T-2 68964 T-3 97642 T-1 6504 T-2 100603 T-3 6989 T-1 1313 T-2 0 T-3 183741 T-1 233 T-2 5081 T-3 164367 ..
.
方法四: 使用可重入鎖, 啟用公平鎖
和3一樣, 但是使用公平鎖, 這種情況下基本上可以做到順序執行, 偶爾會產生多一次循環
private final Lock lock = new ReentrantLock(true);
輸出
T-1 0 T-2 0 T-3 0 T-1 0 T-2 0 T-3 0 T-1 0 T-2 0 T-3 0 T-1 0 T-2 0 T-3 1 T-1 1 T-2 1 T-3 1 ...
.
方法五: 使用Condition
每個線程如果看到不是自己的計數, 就await(), 如果是自己的計數, 就完成打印動作, 再signalAll()所有其他線程去繼續運行, 自己在下一個循環后, 即使又繼續執行, 也會因為計數已經變了而await.
如果ReentrantLock構造參數使用true, 可以基本消除 ~await 這一步的輸出.
public class ReentrantLockCondition2 {
private static Lock lock = new ReentrantLock();
private static Condition condition = lock.newCondition();
private volatile int state = 1;
private void handle(int state) {
lock.lock();
try {
while(true) {
while(this.state != state) {
System.out.println(state + " ~await");
condition.await();
}
System.out.println(state);
this.state = state % 3 + 1;
condition.signalAll();
System.out.println(state + " await");
condition.await();
}
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public static void main(String[] args) {
ReentrantLockCondition2 rlc = new ReentrantLockCondition2();
new Thread(()->rlc.handle(1)).start();
new Thread(()->rlc.handle(2)).start();
new Thread(()->rlc.handle(3)).start();
}
}
.
方法六: 使用多個Condition
給每個線程不同的condition. 這個和4的區別是, 可以用condition.signal()精確地通知對應的線程繼續執行(在對應的condition上await的線程, 可能是多個). 這種情況下是可以多個線程都不unlock鎖的情況下進行協作的. 注意下面的while(true)循環是在lock.lock()方法內部的.
public class ReentrantLockCondition {
private static Lock lock = new ReentrantLock();
private static Condition[] conditions = {lock.newCondition(), lock.newCondition(), lock.newCondition()};
private volatile int state = 1;
private void handle(int state) {
lock.lock();
try {
while(true) {
while(this.state != state) {
conditions[state - 1].await();
}
System.out.println(state);
this.state = state % 3 + 1;
conditions[this.state - 1].signal();
conditions[state - 1].await();
}
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public static void main(String[] args) {
ReentrantLockCondition rlc = new ReentrantLockCondition();
new Thread(()->rlc.handle(1)).start();
new Thread(()->rlc.handle(2)).start();
new Thread(()->rlc.handle(3)).start();
}
}
.