转自：https://blog.csdn.net/dataiyangu/article/details/87631574
多线程调试的方法
使用Eclipse进行多线程调试
线程dump及分析
分析死锁案例
代码
jstack调试
jps命令找到当前这个java的进程号
运行jstack命令
JDK8对并发的新支持
LongAdder
CompletableFuture
基本
异步执行
工厂方法：
流式调用
组合多个CompletableFuture
StampedLock
StampedLock的实现思想
多线程调试的方法
使用Eclipse进行多线程调试
看如下一段代码：
public class UnsafeArrayList {
static ArrayList al=new ArrayList();
static class AddTask implements Runnable{
@Override
public void run() {
try {
Thread.sleep(100);
} catch (InterruptedException e) {}
for(int i=0;i<1000000;i++)
al.add(new Object());
}
}
public static void main(String[] args) throws InterruptedException {
Thread t1=new Thread(new AddTask(),"t1");
Thread t2=new Thread(new AddTask(),"t2");
t1.start();
t2.start();
Thread t3=new Thread(new Runnable(){
@Override
public void run() {
while(true){
try {
Thread.sleep(1000);
} catch (InterruptedException e) {}
}
}
},"t3");
t3.start();
}
}
ArrayList不是线程安全的。
把断点打到ArrayList的add方法处，发现还是在classLoader层面上的，并没有到达我们的应用层的实现。
上面的条件断点只有当不是主线程的时候才会生效，通过上面的程序不难看出，整个应用层面和主线程并没有太大的关系，主要和线程t1 t2有关系
通过打断点的方式复现问题发现
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
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是在ensureCapacityInternal(size + 1);这行出现了问题，t1中size变成了9，t2size++，这个时候t1并不知情，导致size不一致，·8导致报错。
线程dump及分析
jstack 3992 可以导出当前虚拟机所有运行的线程。
在%JAVA_HOME%/bin目录下面（jstack 3992 ）
分析死锁案例
代码
代码简介：东西南北四个小车形成的死锁
import java.util.concurrent.locks.ReentrantLock;
public class DeadLock extends Thread {
protected Object myDirect;
static ReentrantLock south = new ReentrantLock();
static ReentrantLock north = new ReentrantLock();
static ReentrantLock west = new ReentrantLock();
static ReentrantLock east = new ReentrantLock();
public DeadLock(Object obj){
this.myDirect = obj;
if (myDirect == south) {
this.setName("south");
}
if (myDirect == north) {
this.setName("north");
}
if (myDirect == west) {
this.setName("west");
}
if (myDirect == east) {
this.setName("east");
}
}
@Override
public void run() {
if (myDirect == south) {
try {
west.lockInterruptibly();
Thread.sleep(500);
south.lockInterruptibly();
System.out.println("car to south has passed");
} catch (InterruptedException e) {
e.printStackTrace();
System.out.println("car to south is killed");
}finally {
if (west.isHeldByCurrentThread())
west.unlock();
if (south.isHeldByCurrentThread())
south.unlock();
}
}
if (myDirect == north) {
try {
east.lockInterruptibly();
Thread.sleep(500);
north.lockInterruptibly();
System.out.println("car to south has passed");
} catch (InterruptedException e) {
e.printStackTrace();
System.out.println("car to south is killed");
}finally {
if (east.isHeldByCurrentThread())
east.unlock();
if (north.isHeldByCurrentThread())
north.unlock();
}
}
if (myDirect == west) {
try {
north.lockInterruptibly();
Thread.sleep(500);
west.lockInterruptibly();
System.out.println("car to south has passed");
} catch (InterruptedException e) {
e.printStackTrace();
System.out.println("car to south is killed");
}finally {
if (north.isHeldByCurrentThread())
north.unlock();
if (west.isHeldByCurrentThread())
west.unlock();
}
}
if (myDirect == east) {
try {
south.lockInterruptibly();
Thread.sleep(500);
east.lockInterruptibly();
System.out.println("car to south has passed");
} catch (InterruptedException e) {
e.printStackTrace();
System.out.println("car to south is killed");
}finally {
if (south.isHeldByCurrentThread())
south.unlock();
if (east.isHeldByCurrentThread())
east.unlock();
}
}
}
public static void main(String[] args) throws InterruptedException {
DeadLock car2South = new DeadLock(south);
DeadLock car2North = new DeadLock(north);
DeadLock car2West = new DeadLock(west);
DeadLock car2East = new DeadLock(east);
car2South.start();
car2East.start();
car2North.start();
car2West.start();
Thread.sleep(1000);
}
}
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运行结果：
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什么也没有输出，程序还在不断的运行着。
jstack调试
jps命令找到当前这个java的进程号
➜ ~ jps
1682 Launcher
1714 Jps
1683 DeadLock
1397 RemoteMavenServer
1370
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运行jstack命令
jstack 1683
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jstack -h
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发现-l参数可以看到更多的参数
jstack -l 1683
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结果：
"west" #12 prio=5 os_prio=31 tid=0x00007ff6cc062000 nid=0x3d03 waiting on condition [0x0000700006ab7000]
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x000000079578bda8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireInterruptibly(AbstractQueuedSynchronizer.java:897)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireInterruptibly(AbstractQueuedSynchronizer.java:1222)
at java.util.concurrent.locks.ReentrantLock.lockInterruptibly(ReentrantLock.java:335)
at DeadLock.run(DeadLock.java:65)
Locked ownable synchronizers:
- <0x000000079578bd78> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"north" #11 prio=5 os_prio=31 tid=0x00007ff6cd843800 nid=0x3f03 waiting on condition [0x00007000069b4000]
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x000000079578bd78> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireInterruptibly(AbstractQueuedSynchronizer.java:897)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireInterruptibly(AbstractQueuedSynchronizer.java:1222)
at java.util.concurrent.locks.ReentrantLock.lockInterruptibly(ReentrantLock.java:335)
at DeadLock.run(DeadLock.java:49)
Locked ownable synchronizers:
- <0x000000079578bdd8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"east" #13 prio=5 os_prio=31 tid=0x00007ff6cd843000 nid=0x4103 waiting on condition [0x00007000068b1000]
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x000000079578bdd8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireInterruptibly(AbstractQueuedSynchronizer.java:897)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireInterruptibly(AbstractQueuedSynchronizer.java:1222)
at java.util.concurrent.locks.ReentrantLock.lockInterruptibly(ReentrantLock.java:335)
at DeadLock.run(DeadLock.java:81)
Locked ownable synchronizers:
- <0x000000079578bd48> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"south" #10 prio=5 os_prio=31 tid=0x00007ff6cd842000 nid=0x3b03 waiting on condition [0x00007000067ae000]
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x000000079578bd48> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireInterruptibly(AbstractQueuedSynchronizer.java:897)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireInterruptibly(AbstractQueuedSynchronizer.java:1222)
at java.util.concurrent.locks.ReentrantLock.lockInterruptibly(ReentrantLock.java:335)
at DeadLock.run(DeadLock.java:33)
Locked ownable synchronizers:
- <0x000000079578bda8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
- Found one Java-level deadlock:
=============================
"west":
waiting for ownable synchronizer 0x000000079578bda8, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
which is held by "south"
"south":
waiting for ownable synchronizer 0x000000079578bd48, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
which is held by "east"
"east":
waiting for ownable synchronizer 0x000000079578bdd8, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
which is held by "north"
"north":
waiting for ownable synchronizer 0x000000079578bd78, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
which is held by "west"
Java stack information for the threads listed above:
===================================================
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可以看到east中有这句话：parking to wait for <0x000000079578bdd8>
south中Locked ownable synchronizers:
- <0x000000079578bda8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
可以知道east在等待0x000000079578bdd8，而0x000000079578bdd8是被south持有的。以此类推。
同样
“west”:
waiting for ownable synchronizer 0x000000079578bda8, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
which is held by “south”
也是能看出具体的原因。
末尾更清楚：
=============================
"west":
waiting for ownable synchronizer 0x000000079578bda8, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
which is held by "south"
"south":
waiting for ownable synchronizer 0x000000079578bd48, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
which is held by "east"
"east":
waiting for ownable synchronizer 0x000000079578bdd8, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
which is held by "north"
"north":
waiting for ownable synchronizer 0x000000079578bd78, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
which is held by "west"
Java stack information for the threads listed above:
===================================================
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JDK8对并发的新支持
LongAdder
– 和AtomicInteger类似的使用方式
– 在AtomicInteger上进行了热点分离
– public void add(long x)
– public void increment()增加一
– public void decrement()减一
– public long sum() 因为是分离成16份，这里是一个求和的操作
– public long longValue() 同上
– public int intValue() Long转化成整形
性能比AtomicLong高很多，因为LongAdder是类似于HashMao的热点分离。
示意：
cas更新
线程一-------->cell1 |
线程二-------->cell2 |---sum---->
线程三-------->cell3 |----求和---> value
线程四-------->cell4 |
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基本思想：
如上，当高并发的时候，将一个数分解成多个cell，线程一访问cell1，线程二访问cell2，以此类推，从而减少冲突的概率，但是当并发的线程数极少的时候，将数分成数组，则会消耗很大的性能，起到相反的作用，所以Longadd本身是有优化的，本身通过base数据（类似于AtomicLong），当发现一次冲突的时候就分成两个，在发现一次冲突分成四个，以此类推。
CompletableFuture
基本
– 实现CompletionStage接口(40余个方法)
– Java 8中对Future的增强版
– 支持流式调用
stage.thenApply(x -> square(x)).thenAccept(x -> System.out.print(x)).thenRun(() ->
System.out.println())
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完成后得到通知
public static class AskThread implements Runnable {
CompletableFuture <Integer> re = null;
public AskThread(CompletableFuture <Integer> re) {
this.re = re
}
@Override
public void run() [
int myRe = 0;
try {
//返回future值的平方
myRe = re.get) * re.get();
} catch (Exception e) {
System.out.println(myRe);
}
public static void main(String[] args) throws InterruptedException {
final CompletableFuture <Integer> future = new CompletableFuture<>();
//将future传到线程中
new Thread(new AskThread(future)).start();
//模拟长时间的计算过程
Thread.sleep(1000);
//告知完成结果
future.complete(60);
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跟前面的future模式不同的是，前面的future模式完成是系统自己完成的，这里的完成是能够开发者自己定义的，如上面的代码future.complete(60);
异步执行
public static Integer calc(Integer para) {
try {
// 模拟一个长时间的执行
Thread.sleep(1000);
} catch (InterruptedException e) {
}
return para*para;
}
public static void main(String[] args) throws InterruptedException, ExecutionException {
final CompletableFuture<Integer> future =
//supplyAsync工厂方法，能够得到一个CompletableFuture的实例，
//并不是通过new出来的，内部会帮我们创建一个，能够直接得到一个实例，然后调动calc
//calc中的执行类似上面的代码，return平法。
CompletableFuture.supplyAsync(() -> calc(50));
System.out.println(future.get());
}
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工厂方法：
static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier);
static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier, Executor executor); static CompletableFuture<Void> runAsync(Runnable runnable);
static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor);
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Executor executor就是线程池，supplyAsync和runAsync的区别是supplyAsync是有返回值的，runAsync就是一个单纯Runnable接口，没有返回值。
流式调用
public static Integer calc(Integer para) {
try {
// 模拟一个长时间的执行
Thread.sleep(1000);
} catch (InterruptedException e) {
}
return para*para;
}
public static void main(String[] args) throws InterruptedException, ExecutionException {
CompletableFuture<Void> fu=CompletableFuture.supplyAsync(() -> calc(50))
.thenApply((i)->Integer.toString(i)) .thenApply((str)->"\""+str+"\"") .thenAccept(System.out::println);
fu.get();
}
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calc返回平方操作，Integer.toString转化成String，thenApply((str)->"""+str+""") 在String两边加引号，thenAccept(System.out::println)输出结果。fu.get(); 看看得到结果了没有。
组合多个CompletableFuture
public <U> CompletableFuture<U> thenCompose(Function<? super T, ? extends CompletionStage<U>> fn)
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public static Integer calc(Integer para) {
return para/2;
}
public static void main(String[] args) throws InterruptedException, ExecutionException {
CompletableFuture<Void> fu =
CompletableFuture.supplyAsync(() -> calc(50)) .thenCompose((i)->CompletableFuture.supplyAsync(() -> calc(i))) .thenApply((str)->"\"" + str + "\"").thenAccept(System.out::println);
fu.get();
}
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thenCompose除以四，就是五十先除以四，再除以四。
结果
"12"
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StampedLock
– 读写锁的改进
– 读不阻塞写
读的时候发生了写，不应该不让写操作，而应该重读。
因为：
当读太多的时候，可能出现写不进去的现象，写饥饿。
stemp时间戳
public class Point {
private double x, y;
private final StampedLock sl = new StampedLock();
void move(double deltaX, double deltaY) { // an exclusively locked method long stamp = sl.writeLock();
try {
x += deltaX;
y += deltaY;
} finally {
sl.unlockWrite(stamp);
}
}
double distanceFromOrigin() { // A read-only method
//tryOptimisticRead乐观读，即上面提到的思想
long stamp = sl.tryOptimisticRead();
double currentX = x, currentY = y;
//验证stemp，如果读的过程中，进行了写操作，返回零或者其他的数，拒绝操作
//如果在读x的过程中修改了y，看到上面的move函数，对sl加锁解锁，每次的stemp值都是不一样的
//和这里的对比
if (!sl.validate(stamp)) {
//如果不支持乐观读，就用最原始的读写锁的方法。
stamp = sl.readLock();
try {
currentX = x;
currentY = y;
} finally {
sl.unlockRead(stamp);
}
}
return Math.sqrt(currentX * currentX + currentY * currentY);
}
}
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StampedLock的实现思想
– CLH自旋锁
– 锁维护一个等待线程队列，所有申请锁，但是没有成功的线程都记录在这个队列中。每一个节点(一个 节点代表一个线程)，保存一个标记位(locked)，用于判断当前线程是否已经释放锁。
– 当一个线程试图获得锁时，取得当前等待队列的尾部节点作为其前序节点。并使用类似如下代码判断前 序节点是否已经成功释放锁:
示意代码：
while (pred.locked) { }
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StampedLock的实现思想
– 不会进行无休止的自旋，会在在若干次自旋后挂起线程
上面（while）只是一个示意的代码，不会无休止的自旋
简单来说就是每次执行自己的时候先看看前面的锁释放了没有，以此类推。
---------------------
作者：Leesin Dong
来源：CSDN
原文：https://blog.csdn.net/dataiyangu/article/details/87631574
版权声明：本文为博主原创文章，转载请附上博文链接！
