一、题目描述
我们提供了一个类：
public class Foo {
public void first() { print(“first”); }
public void second() { print(“second”); }
public void third() { print(“third”); }
}
三个不同的线程 A、B、C 将会共用一个 Foo 实例。
一个将会调用 first() 方法
一个将会调用 second() 方法
还有一个将会调用 third() 方法
请设计修改程序，以确保 second() 方法在 first() 方法之后被执行，third() 方法在 second() 方法之后被执行。
示例 1:
输入: [1,2,3]
输出: “firstsecondthird”
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 first() 方法，线程 B 将会调用 second() 方法，线程 C 将会调用 third() 方法。
正确的输出是 “firstsecondthird”。
示例 2:
输入: [1,3,2]
输出: “firstsecondthird”
解释:
输入 [1,3,2] 表示线程 A 将会调用 first() 方法，线程 B 将会调用 third() 方法，线程 C 将会调用 second() 方法。
正确的输出是 “firstsecondthird”。
二、题解
方法一：使用ReentrantLock
class Foo {
private int number = 1;
private Lock lock = new ReentrantLock();
private Condition condition1 = lock.newCondition();
private Condition condition2 = lock.newCondition();
private Condition condition3 = lock.newCondition();
public Foo() {
number = 1;
}
public void first(Runnable printFirst) throws InterruptedException {
lock.lock();
try{
if(number!=1){
condition1.await();
}
// printFirst() outputs "first". Do not change or remove this line.
printFirst.run();
number = 2;
condition2.signal();
}finally{
lock.unlock();
}
}
public void second(Runnable printSecond) throws InterruptedException {
lock.lock();
try{
if(number != 2){
condition2.await();
}
// printSecond() outputs "second". Do not change or remove this line.
printSecond.run();
number = 3;
condition3.signal();
}finally{
lock.unlock();
}
}
public void third(Runnable printThird) throws InterruptedException {
lock.lock();
try{
if(number != 3)
condition3.await();
// printThird.run() outputs "third". Do not change or remove this line.
printThird.run();
number = 1;
condition1.signal();
}finally{
lock.unlock();
}
}
}
-方法二：使用Atomic
class Foo {
private AtomicInteger firstJobDone = new AtomicInteger(0);
private AtomicInteger secondJobDone = new AtomicInteger(0);
public Foo() {
}
public void first(Runnable printFirst) throws InterruptedException {
// printFirst() outputs "first". Do not change or remove this line.
printFirst.run();
firstJobDone.incrementAndGet();
}
public void second(Runnable printSecond) throws InterruptedException {
while(firstJobDone.get()!=1){
}
// printSecond() outputs "second". Do not change or remove this line.
printSecond.run();
secondJobDone.incrementAndGet();
}
public void third(Runnable printThird) throws InterruptedException {
while(secondJobDone.get()!=1){
}
// printThird.run() outputs "third". Do not change or remove this line.
printThird.run();
}
}
