这是一道经典的笔试题，最近团队招聘，大家又把它翻出来了。这里稍微梳理下几种做法。
synchronized todo
ReentrantLock jdk1.5并发包引入了lock方式，通过ReentrantLock我们可以很方便的进行显式的锁操作，即获取锁和释放锁，对于同一个对象锁而言，同一时刻只可能有一个线程拿到了这个锁，此时其他线程通过lock.lock()来获取对象锁时都会被阻塞，直到这个线程通过lock.unlock()操作释放这个锁后，其他线程才能拿到这个锁。
package reentrantlock;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class LockABC {private static Lock lock = new ReentrantLock();
// 通过JDK5中的Lock锁来保证线程的访问的互斥
private static int state = 0;
//通过state的值来确定是否打印public static void main(String[] args) {
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0;
i < 10;
) {
lock.lock();
try {
while (state % 3 == 0) {
System.out.println("A");
state++;
i++;
}
} finally {
lock.unlock();
}
}
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0;
i < 10;
) {
lock.lock();
try {
while (state % 3 == 1) {
System.out.println("B");
state++;
i++;
}
} finally {
lock.unlock();
}}}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0;
i < 10;
) {
lock.lock();
try {
while (state % 3 == 2) {
System.out.println("C");
state++;
i++;
}
} finally {
lock.unlock();
}}}
}).start();
}
}
ReentrantLock+Condition 生成多个对象锁，通过控制各个对象锁的获取和释放。
package reentrantlock;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class ConditionABC {private static Lock lock = new ReentrantLock();
static Condition conditionA = lock.newCondition();
static Condition conditionB = lock.newCondition();
static Condition conditionC = lock.newCondition();
private static String state = "A";
public static void main(String[] args) {
new Thread(new Runnable() {
@Override
public void run() {
lock.lock();
try {
for (int i = 0;
i < 10;
i++) {
if (!state.equals("A")){
conditionA.await();
}
state = "B";
System.out.println("A");
conditionB.signal();
}
}catch (Exception e){}finally {
lock.unlock();
}
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
lock.lock();
try {
for (int i = 0;
i < 10;
i++) {
if (!state.equals("B")){
conditionB.await();
}
state = "C";
System.out.println("B");
conditionC.signal();
}
}catch (Exception e){}finally {
lock.unlock();
}
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
lock.lock();
try {
for (int i = 0;
i < 10;
i++) {
if (!state.equals("C")){
conditionC.await();
}
state = "A";
System.out.println("C");
conditionA.signal();
}
}catch (Exception e){}finally {
lock.unlock();
}
}
}).start();
}
}
【交替打印ABC的多种方法】
