JDK源码——java.util.concurrent(六)

测试代码:

https://github.com/kevindai007/springboot_houseSearch/tree/master/src/test/java/com/kevindai/juc

CyclicBarrier

咱们首先通过一个demo来了解CyclicBarrier的用法和特点

public class CyclicBarrierTest {
public static void main(String[] args) {
final CyclicBarrier cyclicBarrier = new CyclicBarrier(10);

for (int i = 0; i < 11; i++) {
Runnable run = new Runnable() {
@Override
public void run() {
System.out.println("线程开始" + Thread.currentThread().getName());
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
try {
cyclicBarrier.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
System.out.println("线程开始启动!"  + Thread.currentThread().getName());
}
};

Thread thread = new Thread(run,"Thread" + i);
thread.start();
}
}
}

这里能看到CyclicBarrier会让调用await()的线程等待,把CyclicBarrier的资源获取完之后,所有的线程一起运行.

下面咱们一起看看其源码.先看看构造函数

public CyclicBarrier(int parties) {
this(parties, null);
}
//传入可获取的资源数及资源被获取完时执行的命令
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}

再来看看await()方法

public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}

private int dowait(boolean timed, long nanos)
throws InterruptedException, BrokenBarrierException,
TimeoutException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
//CyclicBarrier可重复使用,用于做判断是否在同一个条件中
final Generation g = generation;
//为true表示已经被打破,抛异常
if (g.broken)
throw new BrokenBarrierException();
//如果线程被中断,那么打破屏障,唤醒屏障前的其他等待线程，抛出异常
if (Thread.interrupted()) {
breakBarrier();
throw new InterruptedException();
}
//剩余资源数减一
int index = --count;
//如果最后一个资源被获取,那么执行barrierCommand,然后唤醒所有线程
if (index == 0) {  // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null)
command.run();
ranAction = true;
//唤醒屏障前其他等待线程,重置count和new generation
nextGeneration();
return 0;
} finally {
if (!ranAction)
breakBarrier();
}
}

//如果还有资源可以被其他线程获取,那么自旋等待
for (;;) {
try {
if (!timed)
trip.await();
else if (nanos > 0L)
nanos = trip.awaitNanos(nanos);
} catch (InterruptedException ie) {
//如果await的线程被中断，检查下generation
if (g == generation && ! g.broken) {
//处于当前generation并且屏障没有被打破,那就打破屏障
breakBarrier();
throw ie;
} else {
Thread.currentThread().interrupt();
}
}
if (g.broken)
throw new BrokenBarrierException();

if (g != generation)
return index;

if (timed && nanos <= 0L) {
breakBarrier();
throw new TimeoutException();
}
}
} finally {
lock.unlock();
}
}

//当资源被减完时调用此方法,让所有等待线程继续执行,重置count,设置一个新的Generation
private void nextGeneration() {
// signal completion of last generation
trip.signalAll();
// set up next generation
count = parties;
generation = new Generation();
}
//打破屏障
private void breakBarrier() {
generation.broken = true;
count = parties;
trip.signalAll();
}

CyclicBarrier的主要方法到这里就分析完了,主要是用了ReentrantLock+Condition+int count组成,没用 AQS;

注意与CountDownLatch的区别:

CountDownLatch是等待所有线程运行完成之后,然后去运行另外一个(或一组)线程;而CyclicBarrier则是一组线程相互等待,当所有线程准备完毕之后,这组线程一起执行,且可以在等待完成后执行一个屏障命令

CountDownLatch只能使用一次,而CyclicBarrier正常结束后调用nextGeneration初始化可以重复使用

ConcurrentHashMap

说到ConcurrentHashMap不得不先说说HashMap,还好原来分析过HashMap的源码,大家看这里,咱们直接看是看源码吧(这个不做demo了,这就是一个线程安全的HashMap用法也基本相似)

昨天电脑上装了换了JDK8,然后发现JDK8中ConcurrenHashMap的代码真的是复杂到家了,果断换成JDK7来研究

先看看看一些重要的属性

//默认初始大小
static final int DEFAULT_INITIAL_CAPACITY = 16;

//负载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;

//segment的个数
static final int DEFAULT_CONCURRENCY_LEVEL = 16;

//最大容量
static final int MAXIMUM_CAPACITY = 1 << 30;

//segment中table的最小容量
static final int MIN_SEGMENT_TABLE_CAPACITY = 2;

//最大segent数量
static final int MAX_SEGMENTS = 1 << 16;

static final int RETRIES_BEFORE_LOCK = 2;
final int segmentMask;

final int segmentShift;

final Segment<K,V>[] segments;

这是主要字段,基本能够理解,下面咱们从构造方法开始开看ConcurrentHashMap的具体流程

public ConcurrentHashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
}

public ConcurrentHashMap(int initialCapacity,
float loadFactor, int concurrencyLevel) {
//参数校验
if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
throw new IllegalArgumentException();
//设置最大segment数量
if (concurrencyLevel > MAX_SEGMENTS)
concurrencyLevel = MAX_SEGMENTS;
// Find power-of-two sizes best matching arguments
int sshift = 0;
int ssize = 1;//segment数量
while (ssize < concurrencyLevel) {
++sshift;
//ssize向左位移一位
ssize <<= 1;
}
this.segmentShift = 32 - sshift;//segment的偏移量
this.segmentMask = ssize - 1;//segment掩码值
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
int c = initialCapacity / ssize;
if (c * ssize < initialCapacity)
++c;
int cap = MIN_SEGMENT_TABLE_CAPACITY;//2,segment大小
while (cap < c)//这里保证每个segment的大小为2的倍数
cap <<= 1;
Segment<K,V> s0 =
new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
(HashEntry<K,V>[])new HashEntry[cap]);
Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];//用ssize初始化segments数组
UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
this.segments = ss;
}

构造方法中主要进行了参数校验,确认了segment的数量和大小,并初始化S0,下面看看put方法

public V put(K key, V value) {
Segment<K,V> s;
//ConcurrentHashMap value不能为Null
if (value == null)
throw new NullPointerException();
//取key的hashcode再来一次hash,2次hash打撒分布,避免冲突
int hash = hash(key);
//计算要存入的segment的下标
int j = (hash >>> segmentShift) & segmentMask;
if ((s = (Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
(segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
//只初始化了s0，这里确保segment存在
s = ensureSegment(j);
return s.put(key, hash, value, false);//掉segment的put
}

//因为只初始化了S0,所以要保证当存放位置不为S0时segment不为空
private Segment<K,V> ensureSegment(int k) {
final Segment<K,V>[] ss = this.segments;
long u = (k << SSHIFT) + SBASE; //计算偏移量
Segment<K,V> seg;
if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
Segment<K,V> proto = ss[0]; //s0不为空null,所以一些参数直接从s0获取
int cap = proto.table.length;
float lf = proto.loadFactor;
int threshold = (int)(cap * lf);
//构造segment里面的table
HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap];
if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
== null) { // recheck
Segment<K,V> s = new Segment<K,V>(lf, threshold, tab);
//自旋+cas保证存储位置一定设置成功
while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
== null) {
if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
break;
}
}
}
return seg;
}

这时咱们来看看Segment是怎么实现的

static final class Segment<K,V> extends ReentrantLock implements Serializable {
//segment中的table
transient volatile HashEntry<K,V>[] table;
//链表长度,即元素数量
transient int count;
//修改次数
transient int modCount;
//极限值,当table中包含的HashEntry元素的个数超过此值时,触发table的再散列
transient int threshold;
//加载因子
final float loadFactor;
Segment(float lf, int threshold, HashEntry<K,V>[] tab) {
this.loadFactor = lf;
this.threshold = threshold;
this.table = tab;
}
//Concurrent的put操作,其实就是找到相应的Sement然后调用此put方法
final V put(K key, int hash, V value, boolean onlyIfAbsent) {
//首先尝试加锁,加锁失败则调用scanAndLockForPut自旋加锁
HashEntry<K,V> node = tryLock() ? null :
scanAndLockForPut(key, hash, value);
V oldValue;
try {
HashEntry<K,V>[] tab = table;
//在table中查找key对应的位置
int index = (tab.length - 1) & hash;
//获取第一个节点
HashEntry<K,V> first = entryAt(tab, index);
for (HashEntry<K,V> e = first;;) {
//节点存在就检查是否存在相同的key,如果存在则覆盖值
if (e != null) {
K k;
if ((k = e.key) == key ||
(e.hash == hash && key.equals(k))) {
oldValue = e.value;
if (!onlyIfAbsent) {
e.value = value;
++modCount;
}
break;
}
e = e.next;
}
else {//不存在就新建一个
if (node != null)
node.setNext(first);
else
node = new HashEntry<K,V>(hash, key, value, first);
int c = count + 1;
//超过长度则rehash
if (c > threshold && tab.length < MAXIMUM_CAPACITY)
rehash(node);
else
setEntryAt(tab, index, node);
++modCount;
count = c;
oldValue = null;
break;
}
}
} finally {
unlock();
}
return oldValue;
}
private void rehash(HashEntry<K,V> node) {
HashEntry<K,V>[] oldTable = table;
int oldCapacity = oldTable.length;
int newCapacity = oldCapacity << 1; //新table大小
threshold = (int)(newCapacity * loadFactor); //新的极限值
HashEntry<K,V>[] newTable =
(HashEntry<K,V>[]) new HashEntry[newCapacity]; //创建新的table数组
int sizeMask = newCapacity - 1; //计算具体位置时用，跟hashmap计算方式一样
for (int i = 0; i < oldCapacity ; i++) { //循环oldtable
HashEntry<K,V> e = oldTable[i];
if (e != null) {
HashEntry<K,V> next = e.next;
int idx = e.hash & sizeMask;
if (next == null)   //  只有一个节点，直接移过去
newTable[idx] = e;
else { // 节点重用
HashEntry<K,V> lastRun = e;
int lastIdx = idx;
//下面2个for循环的逻辑是lastRun，last从next节点往后移，最后lastRun指向最后一个转移到新table的index不变的节点
//比较乱，画图走几遍，意思就是说假如原来的table[1]有10个节点，然后不停计算节点在newtable的位置，很可能从第四个节点的时候开始，
//后面的所有节点在newtable中的存储位置都一样了，那么我newtable只要把第4个节点直接放过去就行，然后从链表头开始处理其他节点，
//就不用把所有节点都新建一遍了
for (HashEntry<K,V> last = next;
last != null;
last = last.next) {
int k = last.hash & sizeMask;
if (k != lastIdx) {
lastIdx = k;
lastRun = last;
}
}
newTable[lastIdx] = lastRun; //直接lastRun设置到newtable
// 复制其他节点
for (HashEntry<K,V> p = e; p != lastRun; p = p.next) {
V v = p.value;
int h = p.hash;
int k = h & sizeMask;
HashEntry<K,V> n = newTable[k];
newTable[k] = new HashEntry<K,V>(h, p.key, v, n);
}
}
}
}
int nodeIndex = node.hash & sizeMask; // 把新节点加入到newtable
node.setNext(newTable[nodeIndex]);
newTable[nodeIndex] = node;
table = newTable;
}

/**
* 自旋尝试加锁，不成功扫描对应位置的链表，如果链表中key不存在就创建一个node，达到最大次数后就阻塞加锁，如果key存在返回的null
* 处理过程中其他线程改变了链表结构，那就重头再来
*/
private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) {
HashEntry<K,V> first = entryForHash(this, hash);
HashEntry<K,V> e = first;
HashEntry<K,V> node = null;
int retries = -1; // negative while locating node
while (!tryLock()) {
HashEntry<K,V> f; // to recheck first below
if (retries < 0) {
if (e == null) { //基本就是查找key不存在就创建一个,存在就trylock一直到次数限制，再不行就阻塞加锁
if (node == null)
node = new HashEntry<K,V>(hash, key, value, null);
retries = 0;
}
else if (key.equals(e.key))
retries = 0;
else
e = e.next;
}
else if (++retries > MAX_SCAN_RETRIES) { //超过最大尝试次数，那么就lock阻塞，单核1，多核64
lock();
break;
}
else if ((retries & 1) == 0 &&
(f = entryForHash(this, hash)) != first) { //隔一次检查一遍尝试的时候发现链表的首节点变化了，也就是有别的线程操作了，那就重来
e = first = f; // re-traverse if entry changed
retries = -1;
}
}
return node;
}
}

可以看到Segment继承了ReentrantLock,因此在put方法中能保证线程安全.通过Segment中比较重要的方法基本就是这些,但其中还有很多看不懂的地方,会继续努力的