如下的几个问题是笔试、面试中经常出现的,也是典型的线程问题。
1、main方法启动不同进程
public class A{
// 定义该类的静态Field
public static int a = 6;
}
public class ATest1{
public static void main(String[] args){
// 创建A类的实例
A a = new A();
// 让a实例的a Field的值自加
a.a ++;
System.out.println(a.a);
}
}
public class ATest2{
public static void main(String[] args) {
// 创建A类的实例
A b = new A();
// 输出b实例的a Field的值
System.out.println(b.a);
}
}
如上的两个main方法会各自启动一个进程,而进程的堆是不共享的,所以互不干扰。a.a的值为7,而b.a的值仍为6
2、多线程与同步
多线程有两种实现方法,分别是继承Thread类与实现Runnable接口同步的实现方面有两种,分别是synchronized,wait与notify
3、当一个线程进入一个对象的一个synchronized方法后,其它线程是否可进入此对象的其它方法
分几种情况:
1.其他方法前是否加了synchronized关键字,如果没加,则能。
2.如果这个方法内部调用了wait,则可以进入其他synchronized方法。
3.如果其他个方法都加了synchronized关键字,并且内部没有调用wait,则不能。
4.如果其他方法是static,它用的同步锁是当前类的字节码,与非静态的方法不能同步,因为非静态的方法用的是this。所以这里是可以进入的
4、设计4个线程,其中两个线程每次对j增加1,另外两个线程对j每次减少1
public class test3 {
private int j;
public static void main(String args[]) {
test3 tt = new test3();
Inc inc = tt.new Inc();
Dec dec = tt.new Dec();
for (int i = 0; i < 2; i++) {
Thread t = new Thread(inc);
t.start();
t = new Thread(dec);
t.start();
}
}
private synchronized void inc() {
j++;
System.out.println(Thread.currentThread().getName() + "-inc:" + j);
}
private synchronized void dec() {
j--;
System.out.println(Thread.currentThread().getName() + "-dec:" + j);
}
class Inc implements Runnable {
public void run() {
for (int i = 0; i < 100; i++) {
inc();
}
}
}
class Dec implements Runnable {
public void run() {
for (int i = 0; i < 100; i++) {
dec();
}
}
}
}
或者也可以使用如下的实现方式:
class JManager {
private int j = 0;
public synchronized void subtract() {
j--;
}
public synchronized void accumulate() {
j++;
}
}
class A {
JManager j = new JManager();
void main() {
new A().call();
}
void call() {
for (int i = 0; i < 2; i++) {
new Thread(new Runnable() {
public void run() {
while (true) {
j.accumulate();
}
}
}).start();
new Thread(new Runnable() {
public void run() {
while (true) {
j.subtract();
}
}
}).start();
}
}
}
5、子线程循环10次,接着主线程循环100,接着又回到子线程循环10次,接着再回到主线程又循环100,如此循环50次(这个题目类似与生产者-消费者的实现逻辑)
public class test3 {
private static boolean bShouldMain = false;
public static void main(String[] args) {
new Thread(new Runnable() {
public void run() {
for (int i = 0; i < 50; i++) {
synchronized (test3.class) {
if (bShouldMain) {
try {
test3.class.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
for (int j = 0; j < 10; j++) {
System.out.println(Thread.currentThread().getName()
+ "i=" + i + ",j=" + j);
}
bShouldMain = true;
test3.class.notify();
}
}
}
}).start();
for (int i = 0; i < 50; i++) {
synchronized (test3.class) {
if (!bShouldMain) {
try {
test3.class.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
for (int j = 0; j < 5; j++) {
System.out.println(Thread.currentThread().getName() + "i="
+ i + ",j=" + j);
}
bShouldMain = false;
test3.class.notify();
}
}
}
}
或者还可以使用线程池来实现。具体代码如下:
import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.Condition;
public class test3 {
private static Lock lock = new ReentrantLock();
private static Condition subThreadCondition = lock.newCondition();
private static boolean bBhouldSubThread = false;
public static void main(String[] args) {
ExecutorService threadPool = Executors.newFixedThreadPool(3);
threadPool.execute(new Runnable() {
public void run() {
for (int i = 0; i < 50; i++) {
lock.lock();
try {
if (!bBhouldSubThread)
subThreadCondition.await();
for (int j = 0; j < 10; j++) {
System.out.println(Thread.currentThread().getName()
+ ",j=" + j);
}
bBhouldSubThread = false;
subThreadCondition.signal();
} catch (Exception e) {
} finally {
lock.unlock();
}
}
}
});
threadPool.shutdown();
for (int i = 0; i < 50; i++) {
lock.lock();
try {
if (bBhouldSubThread)
subThreadCondition.await();
for (int j = 0; j < 10; j++) {
System.out.println(Thread.currentThread().getName() + ",j="
+ j);
}
bBhouldSubThread = true;
subThreadCondition.signal();
} catch (Exception e) {
} finally {
lock.unlock();
}
}
}
}