關(guān)于線成暫停和恢復(fù)的那些事兒
suspend()和resume()
這是一對(duì)相反的操作兼吓，suspend可以掛起當(dāng)前線成，而resume可以恢復(fù)當(dāng)前線成攒钳。 乍一看這對(duì)操作非常有用挽荠，但是如果對(duì)jdk源碼稍微有點(diǎn)了解，會(huì)發(fā)現(xiàn)這兩個(gè)方法早就廢棄了德挣。
為什么會(huì)廢棄呢恭垦？
因?yàn)槿绻鹯esume操作，出現(xiàn)在suspend之前的話格嗅，線成會(huì)永久阻塞番挺，會(huì)影響到后面訪問(wèn)當(dāng)前臨界區(qū)的所有線成。最誤導(dǎo)人的是屯掖，當(dāng)前線成的狀態(tài)是runnable玄柏。
suspend問(wèn)題演示
package com.allen.dayup.高并發(fā)程序設(shè)計(jì).chap1;
/**
* @Auther: allen
* @Date: 2020-03-19 21:16
* @Description:
*/
public class BadSuspend {
static Object u = new Object();
static ChangeObjectThread t1 = new ChangeObjectThread("t1");
static ChangeObjectThread t2 = new ChangeObjectThread("t2");
public static class ChangeObjectThread extends Thread{
public ChangeObjectThread(String name) {
super.setName(name);
}
@Override
public void run() {
synchronized (u){
System.out.println("in " + Thread.currentThread().getName());
Thread.currentThread().suspend();
}
}
}
public static void main(String[] args) throws InterruptedException{
//對(duì)照觀察t1和t2，
//休眠主線程讓t1阻塞：t1線成是先調(diào)用suspend懂扼，然后再resume
//主線程啟動(dòng)t2后直接調(diào)用resume：t2線程的resume會(huì)在suspend之前調(diào)用
t1.start();
//主線程休眠100毫秒禁荸，保證t1已經(jīng)阻塞
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
t1.resume();
t2.resume();
//等待t1和t2執(zhí)行完成后，主線程再繼續(xù)執(zhí)行
t1.join();
t2.join();
}
}
運(yùn)行程序會(huì)發(fā)現(xiàn)主線程會(huì)一直阻塞阀湿，用jstack查看線成信息發(fā)現(xiàn)t2線成是runnable狀態(tài)
C:\Users\allen>jstack 20336
2020-04-18 09:11:31
Full thread dump OpenJDK 64-Bit Server VM (25.161-b14 mixed mode):
"t2" #15 prio=5 os_prio=0 tid=0x000000001f4a60f0 nid=0x42f0 runnable [0x00000000203af000]
java.lang.Thread.State: RUNNABLE
at java.lang.Thread.suspend0(Native Method)
at java.lang.Thread.suspend(Thread.java:1032)
at com.allen.dayup.高并發(fā)程序設(shè)計(jì).chap1.BadSuspend$ChangeObjectThread.run(BadSuspend.java:26)
- locked <0x000000076b8d2618> (a java.lang.Object)
"Service Thread" #13 daemon prio=9 os_prio=0 tid=0x000000001f387510 nid=0x56c0 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C1 CompilerThread3" #12 daemon prio=9 os_prio=2 tid=0x000000001f2efab0 nid=0x34a8 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread2" #11 daemon prio=9 os_prio=2 tid=0x000000001f2ef630 nid=0x256c waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread1" #10 daemon prio=9 os_prio=2 tid=0x000000001f2ef1b0 nid=0x4c90 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread0" #9 daemon prio=9 os_prio=2 tid=0x00000000035164f0 nid=0x5530 waiting on condition [0x0000000000000000]
利用wait和notify赶熟，用一個(gè)鉤子方法來(lái)阻塞和恢復(fù)線成
public class GoodSuspend {
static Object lock = new Object();
static class ChangeObjectThread extends Thread {
volatile boolean suspendMe = false;
public void suspendMe(){
this.suspendMe = true;
}
public void resumeMe(){
suspendMe = false;
synchronized (this){
notify();
}
}
@Override
public void run() {
while (true){
//如果suspendMe為true，調(diào)用wait方法阻塞當(dāng)前線成陷嘴；否則映砖，就執(zhí)行業(yè)務(wù)邏輯
synchronized (this) {
while (suspendMe) {
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized (lock) {
System.out.println("in ChangeObjectThread!");
}
Thread.yield();
}
}
}
}
static class ReadObjectThread extends Thread{
@Override
public void run() {
while (true){
synchronized (lock){
System.out.println("in ReadObjectLock!");
Thread.yield();
}
}
}
}
public static void main(String[] args) throws InterruptedException{
ChangeObjectThread t1 = new ChangeObjectThread();
ReadObjectThread t2 = new ReadObjectThread();
t1.start();
t2.start();
Thread.sleep(1000);
t1.suspendMe();
System.out.println("=====================》suspend t1 2 second");
Thread.sleep(2000);
System.out.println("resume t1");
t1.resumeMe();
}
}
- 啟動(dòng)t1和t2線成，t1和t2交替輸出一秒灾挨。
- 然后阻塞t1兩秒邑退，就只有t2線成輸出
- 最后恢復(fù)t1，又重新交替輸出
park()和unpark()
不過(guò)對(duì)于線成暫停和恢復(fù)劳澄，再jdk的并發(fā)包中提供了LockSupport的處理地技。
- park():阻塞線程
- unpark():恢復(fù)線程
在park實(shí)現(xiàn)中，阻塞線成之前先去請(qǐng)求一個(gè)許可秒拔，請(qǐng)求到才能阻塞線成莫矗。只有線成unpark后，許可才會(huì)變成可用狀態(tài)。
所以就算unpark發(fā)生在park之前作谚，也不會(huì)導(dǎo)致線成永久掛起三娩。
public class LockSupportDemo {
static Object u = new Object();
static ChangeObjectThread t1 = new ChangeObjectThread("t1");
static ChangeObjectThread t2 = new ChangeObjectThread("t2");
static class ChangeObjectThread extends Thread {
public ChangeObjectThread(String name) {
super(name);
}
@Override
public void run() {
synchronized (u){
System.out.println("in " + getName());
LockSupport.park();
}
}
}
public static void main(String[] args) throws InterruptedException {
t1.start();
Thread.sleep(100);
t2.start();
LockSupport.unpark(t1);
LockSupport.unpark(t2);
t1.join();
t2.join();
}
}
這段代碼只是將suspend和resume，替換成了park和unpark妹懒。執(zhí)行結(jié)果會(huì)發(fā)現(xiàn)線成正常運(yùn)行結(jié)束雀监。
參考文檔
實(shí)戰(zhàn)java高并發(fā)程序 葛一鳴
