我们提供了一个类：
public class Foo {
public void one() { print("one"); }
public void two() { print("two"); }
public void three() { print("three"); }
}
三个不同的线程将会共用一个 Foo 实例。
线程 A 将会调用 one() 方法
线程 B 将会调用 two() 方法
线程 C 将会调用 three() 方法
请设计修改程序，以确保 two() 方法在 one() 方法之后被执行，three() 方法在 two() 方法之后被执行。
示例 1:
输入: [1,2,3]
输出: "onetwothree"
解释:
有三个线程会被异步启动。
输入 [1,2,3] 表示线程 A 将会调用 one() 方法，线程 B 将会调用 two() 方法，线程 C 将会调用 three() 方法。
正确的输出是 "onetwothree"。
示例 2:
输入: [1,3,2]
输出: "onetwothree"
解释:
输入 [1,3,2] 表示线程 A 将会调用 one() 方法，线程 B 将会调用 three() 方法，线程 C 将会调用 two() 方法。
正确的输出是 "onetwothree"。
注意:
尽管输入中的数字似乎暗示了顺序，但是我们并不保证线程在操作系统中的调度顺序。
你看到的输入格式主要是为了确保测试的全面性。
方法1：volatile
class Foo {
volatile int num = 0;
public Foo() {
}
public void first(Runnable printFirst) throws InterruptedException {
while(num!=0){
}
// printFirst.run() outputs "first". Do not change or remove this line.
printFirst.run();
num++;
}
public void second(Runnable printSecond) throws InterruptedException {
while(num!=1){
}
// printSecond.run() outputs "second". Do not change or remove this line.
printSecond.run();
num++;
}
public void third(Runnable printThird) throws InterruptedException {
while(num!=2){
}
// printThird.run() outputs "third". Do not change or remove this line.
printThird.run();
}
}方法2：synchronize wait notifyAll
class Foo {
Object lock = new Object();
boolean fisrtDone = false;
boolean secondDone = false;
public Foo() {
}
public void first(Runnable printFirst) throws InterruptedException {
synchronized(lock){
// printFirst.run() outputs "first". Do not change or remove this line.
printFirst.run();
fisrtDone = true;
lock.notifyAll();
}
}
public void second(Runnable printSecond) throws InterruptedException {
synchronized(lock){
while(!fisrtDone){
lock.wait();
}
// printSecond.run() outputs "second". Do not change or remove this line.
printSecond.run();
secondDone = true;
lock.notifyAll();
}
}
public void third(Runnable printThird) throws InterruptedException {
synchronized(lock){
while(!secondDone){
lock.wait();
}
// printThird.run() outputs "third". Do not change or remove this line.
printThird.run();
}
}
}思路3：AtomInteger
class Foo {
private AtomicInteger firstJobDone = new AtomicInteger(0);
private AtomicInteger secondJobDone = new AtomicInteger(0);
public Foo() {}
public void first(Runnable printFirst) throws InterruptedException {
// printFirst.run() outputs "first".
printFirst.run();
// mark the first job as done, by increasing its count.
firstJobDone.incrementAndGet();
}
public void second(Runnable printSecond) throws InterruptedException {
while (firstJobDone.get() != 1) {
// waiting for the first job to be done.
}
// printSecond.run() outputs "second".
printSecond.run();
// mark the second as done, by increasing its count.
secondJobDone.incrementAndGet();
}
public void third(Runnable printThird) throws InterruptedException {
while (secondJobDone.get() != 1) {
// waiting for the second job to be done.
}
// printThird.run() outputs "third".
printThird.run();
}
}
思路4：ConutDownLatch
class Foo {
CountDownLatch first = new CountDownLatch(1);
CountDownLatch second = new CountDownLatch(1);
public Foo() {}
public void first(Runnable printFirst) throws InterruptedException {
// printFirst.run() outputs "first".
printFirst.run();
first.countDown();
}
public void second(Runnable printSecond) throws InterruptedException {
first.await();
// printSecond.run() outputs "second".
printSecond.run();
second.countDown();
}
public void third(Runnable printThird) throws InterruptedException {
second.await();
// printThird.run() outputs "third".
printThird.run();
}
}