Java高併發程序設計學習筆記（十）：併發調試和JDK8新特性

轉自：https://blog.csdn.net/dataiyangu/article/details/87631574

多線程調試的方法
使用Eclipse進行多線程調試
線程dump及分析
分析死鎖案例
代碼
jstack調試
jps命令找到當前這個java的進程號
運行jstack命令
JDK8對併發的新支持
LongAdder
CompletableFuture
基本
異步執行
工廠方法：
流式調用
組合多個CompletableFuture
StampedLock
StampedLock的實現思想
多線程調試的方法
使用Eclipse進行多線程調試
看以下一段代碼：

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不是線程安全的。

把斷點打到ArrayList的add方法處，發現仍是在classLoader層面上的，並無到達咱們的應用層的實現。

上面的條件斷點只有當不是主線程的時候纔會生效，經過上面的程序不難看出，整個應用層面和主線程並無太大的關係，主要和線程t1 t2有關係

 

經過打斷點的方式復現問題發現

public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
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是在ensureCapacityInternal(size + 1);這行出現了問題，t1中size變成了9，t2size++，這個時候t1並不知情，致使size不一致，·8致使報錯。

線程dump及分析
jstack 3992 能夠導出當前虛擬機全部運行的線程。
在%JAVA_HOME%/bin目錄下面（jstack 3992 ）

分析死鎖案例
代碼
代碼簡介：東西南北四個小車造成的死鎖

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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運行結果：

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什麼也沒有輸出，程序還在不斷的運行着。

jstack調試
jps命令找到當前這個java的進程號
➜ ~ jps
1682 Launcher
1714 Jps
1683 DeadLock
1397 RemoteMavenServer
1370
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運行jstack命令
jstack 1683
1
jstack -h
1
發現-l參數能夠看到更多的參數

jstack -l 1683
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結果：

"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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能夠看到east中有這句話：parking to wait for <0x000000079578bdd8>
south中Locked ownable synchronizers:
- <0x000000079578bda8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
能夠知道east在等待0x000000079578bdd8，而0x000000079578bdd8是被south持有的。以此類推。

一樣
「west」:
waiting for ownable synchronizer 0x000000079578bda8, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
which is held by 「south」
也是能看出具體的緣由。

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對併發的新支持
LongAdder
– 和AtomicInteger相似的使用方式
– 在AtomicInteger上進行了熱點分離
– public void add(long x)
– public void increment()增長一
– public void decrement()減一
– public long sum() 由於是分離成16份，這裏是一個求和的操做
– public long longValue() 同上
– public int intValue() Long轉化成整形
性能比AtomicLong高不少，由於LongAdder是相似於HashMao的熱點分離。
示意：

cas更新
線程一-------->cell1 |
線程二-------->cell2 |---sum---->
線程三-------->cell3 |----求和---> value
線程四-------->cell4 |
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基本思想：
如上，當高併發的時候，將一個數分解成多個cell，線程一訪問cell1，線程二訪問cell2，以此類推，從而減小衝突的機率，可是當併發的線程數極少的時候，將數分紅數組，則會消耗很大的性能，起到相反的做用，因此Longadd自己是有優化的，自己經過base數據（相似於AtomicLong），當發現一次衝突的時候就分紅兩個，在發現一次衝突分紅四個，以此類推。

CompletableFuture
基本
– 實現CompletionStage接口(40餘個方法)
– Java 8中對Future的加強版
– 支持流式調用

stage.thenApply(x -> square(x)).thenAccept(x -> System.out.print(x)).thenRun(() ->
System.out.println())
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完成後獲得通知

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<>();
//將future傳到線程中
new Thread(new AskThread(future)).start();
//模擬長時間的計算過程
Thread.sleep(1000);
//告知完成結果
future.complete(60);
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跟前面的future模式不一樣的是，前面的future模式完成是系統本身完成的，這裏的完成是可以開發者本身定義的，如上面的代碼future.complete(60);

異步執行
public static Integer calc(Integer para) {
try {
// 模擬一個長時間的執行
Thread.sleep(1000);
} catch (InterruptedException e) {
}
return para*para;
}
public static void main(String[] args) throws InterruptedException, ExecutionException {
final CompletableFuture<Integer> future =
//supplyAsync工廠方法，可以獲得一個CompletableFuture的實例，
//並非經過new出來的，內部會幫咱們建立一個，可以直接獲得一個實例，而後調動calc
//calc中的執行相似上面的代碼，return平法。
CompletableFuture.supplyAsync(() -> calc(50));
System.out.println(future.get());
}
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工廠方法：
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就是線程池，supplyAsync和runAsync的區別是supplyAsync是有返回值的，runAsync就是一個單純Runnable接口，沒有返回值。

流式調用
public static Integer calc(Integer para) {
try {
// 模擬一個長時間的執行
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返回平方操做，Integer.toString轉化成String，thenApply((str)->"""+str+""") 在String兩邊加引號，thenAccept(System.out::println)輸出結果。fu.get(); 看看獲得結果了沒有。

組合多個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。
結果

"12"
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StampedLock
– 讀寫鎖的改進
– 讀不阻塞寫
讀的時候發生了寫，不該該不讓寫操做，而應該重讀。
由於：
當讀太多的時候，可能出現寫不進去的現象，寫飢餓。
stemp時間戳

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樂觀讀，即上面提到的思想
long stamp = sl.tryOptimisticRead();
double currentX = x, currentY = y;
//驗證stemp，若是讀的過程當中，進行了寫操做，返回零或者其餘的數，拒絕操做
//若是在讀x的過程當中修改了y，看到上面的move函數，對sl加鎖解鎖，每次的stemp值都是不同的
//和這裏的對比
if (!sl.validate(stamp)) {
//若是不支持樂觀讀，就用最原始的讀寫鎖的方法。
stamp = sl.readLock();
try {
currentX = x;
currentY = y;
} finally {
sl.unlockRead(stamp);
}
}
return Math.sqrt(currentX * currentX + currentY * currentY);
}
}
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StampedLock的實現思想
– CLH自旋鎖
– 鎖維護一個等待線程隊列，全部申請鎖，可是沒有成功的線程都記錄在這個隊列中。每個節點(一個 節點表明一個線程)，保存一個標記位(locked)，用於判斷當前線程是否已經釋放鎖。
– 當一個線程試圖得到鎖時，取得當前等待隊列的尾部節點做爲其前序節點。並使用相似以下代碼判斷前 序節點是否已經成功釋放鎖:
示意代碼：

while (pred.locked) { }
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StampedLock的實現思想
– 不會進行無休止的自旋，會在在若干次自旋後掛起線程
上面（while）只是一個示意的代碼，不會無休止的自旋

簡單來講就是每次執行本身的時候先看看前面的鎖釋放了沒有，以此類推。