青春须早为，岂能长少年
Collections.synchronizedXXX
线程安全包装类是怎么实现的?
ArrayList list = new ArrayList<>();
List integers = Collections.synchronizedList(list);
ArrayList 实现了RandomAccess
public class ArrayList extends AbstractList
implements List, RandomAccess, Cloneable, java.io.Serializable
...
Collections.synchronizedList 是这么实现的：
//根据指定的list返回一个线程安全的list。
public static List synchronizedList(List list) {
return (list instanceof RandomAccess ?
new SynchronizedRandomAccessList<>(list) :
new SynchronizedList<>(list));
}
然后去看看SynchronizedRandomAccessList,这个是一个静态内部类来着，父类是SynchronizedList
static class SynchronizedRandomAccessList
extends SynchronizedList
implements RandomAccess {
SynchronizedRandomAccessList(List list) {
super(list);
}
private static final long serialVersionUID = 1530674583602358482L;
// 省略...
}
去看看父类SynchronizedList
/**
* @serial include
*/
static class SynchronizedList
extends SynchronizedCollection
implements List {
private static final long serialVersionUID = -7754090372962971524L;
final List list;
SynchronizedList(List list) {
super(list);
this.list = list;
}
SynchronizedList(List list, Object mutex) {
super(list, mutex);
this.list = list;
}
//省略...
}
可以看到还是用synchronized来实现的，那么mutex这把锁默认是什么呢？
去看一下父类：SynchronizedCollection
/**
* @serial include
*/
static class SynchronizedCollection implements Collection, Serializable {
private static final long serialVersionUID = 3053995032091335093L;
final Collection c; // Backing Collection
final Object mutex; // Object on which to synchronize
SynchronizedCollection(Collection c) {
this.c = Objects.requireNonNull(c);
mutex = this;
}
SynchronizedCollection(Collection c, Object mutex) {
this.c = Objects.requireNonNull(c);
this.mutex = Objects.requireNonNull(mutex);
}
//省略...
}
好，现在可以知道默认的锁就是this，也就是自己，还可以在创建时订制锁。
一言蔽之，Collections.synchronizedList 是通过把list所有已知的方法都用synchronized关键字包装起来，从而达到串行访问。
ConcurrentHashMap
ConcurrentHashMap 如何实现线程安全？
- 大量的利用了volatile，final，CAS等lock-free技术来减少锁竞争对于性能的影响，
JDK1.7的实现
public V put(K key, V value) {
Segment s;
if (value == null)
throw new NullPointerException();
//得到hash值
int hash = hash(key);
// 获取到段号
int j = (hash >>> segmentShift) & segmentMask;
if ((s = (Segment)UNSAFE.getObject // nonvolatile; recheck
(segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment
//段为空，创建段
s = ensureSegment(j);
//
return s.put(key, hash, value, false);
}
为了保证创建时的线程安全，采用了volatile和CAS操作
private Segment ensureSegment(int k) {
//省略...
if ((seg = (Segment)UNSAFE.getObjectVolatile(ss, u))
== null) { // recheck
Segment s = new Segment(lf, threshold, tab);
while ((seg = (Segment)UNSAFE.getObjectVolatile(ss, u))
== null) {
if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
break;
}
}
final V put(K key, int hash, V value, boolean onlyIfAbsent) {
//尝试获取锁
//tryLock:如果当前线程可以得到锁，返回true
//scanAndLockForPut：
HashEntry node = tryLock() ? null : scanAndLockForPut(key, hash, value);
V oldValue;
try {
HashEntry[] tab = table;
int index = (tab.length - 1) & hash;
HashEntry first = entryAt(tab, index);
for (HashEntry e = first;;) {
//省略...
}
} finally {
//释放锁
unlock();
}
return oldValue;
}
总结
大概总结一下就是：
- 利用volatile：UNSAFE.getObjectVolatile
- 保证读取对象的可见性
- 利用原子操作CAS：UNSAFE.compareAndSwapObject
- 保证写入的原子性
- ConcurrentHashMap并不允许key或者value为null
- UNSAFE是一个面向底层的类，技能比较逆天，相当于c的malloc和指针，可以直接操作内存。
JDK 1.8的实现
- ConcurrentHashMap中有一个内部类Node
static class Node implements Map.Entry {
final int hash;
final K key;
volatile V val;
volatile Node next;
Node(int hash, K key, V val, Node next) {
this.hash = hash;
this.key = key;
this.val = val;
this.next = next;
}
public final K getKey() { return key; }
public final V getValue() { return val; }
public final int hashCode() { return key.hashCode() ^ val.hashCode(); }
public final String toString(){ return key + "=" + val; }
public final V setValue(V value) {
throw new UnsupportedOperationException();
}
public final boolean equals(Object o) {
Object k, v, u; Map.Entry e;
return ((o instanceof Map.Entry) &&
(k = (e = (Map.Entry)o).getKey()) != null &&
(v = e.getValue()) != null &&
(k == key || k.equals(key)) &&
(v == (u = val) || v.equals(u)));
}
这个是线程安全的，也就是说节点是线程安全的
/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null)
throw new NullPointerException();
//获得hash
int hash = spread(key.hashCode());
int binCount = 0;
//table是volitate的，垃圾箱，第一次使用时创建
//transient volatile Node[] table;
for (Node[] tab = table;;) {
Node f;
int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
if (casTabAt(tab, i, null,
new Node(hash, key, value, null)))
break; // no lock when adding to empty bin
}
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
for (Node e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node pred = e;
if ((e = e.next) == null) {
pred.next = new Node(hash, key,
value, null);
break;
}
}
}
else if (f instanceof TreeBin) {
Node p;
binCount = 2;
if ((p = ((TreeBin)f).putTreeVal(hash, key,
value)) != null) {
oldVal = p.val;
if (!onlyIfAbsent)
p.val = value;
}
}
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
}
private transient volatile int sizeCtl;
控制表初始化和resize
- 当sizeCtl<0:
- sizeCtl=-1:table 正在初始化
- sizeCtl<-1: 有 -(sizeCtl+1)个线程在扩容
- 否则：
- 当table==null: 表示初始化的大小,0 或者默认
- table!=null: 表示下次扩容的大小
size()获取大小是估算值，不是实时的，直接拿历史每个槽位的统计值相加
public int size() {
//不加任何锁，直接数一遍，是不精确的
long n = sumCount();
return ((n < 0L) ? 0 :
(n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
(int)n);
}
final long sumCount() {
//当前槽位数
CounterCell[] as = counterCells;
CounterCell a;
long sum = baseCount;
if (as != null) {
//直接把每一个槽位拥有的数量加起来
for (int i = 0; i < as.length; ++i) {
if ((a = as[i]) != null)
sum += a.value;
}
}
return sum;
}
/**
* counterCells 的长度就是表长度
* counterCells[k]=n；表示第n个节点，存在hash相同的元素为n个
* 这n个对象以红黑树存储
*/
private transient volatile CounterCell[] counterCells;
//内部类，volatile保证可见性
@sun.misc.Contended static final class CounterCell {
volatile long value;
CounterCell(long x) { value = x; }
}
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code key.equals(k)},
* then this method returns {@code v}; otherwise it returns
* {@code null}. (There can be at most one such mapping.)
*
* @throws NullPointerException if the specified key is null
*/
public V get(Object key) {
Node[] tab;
Node e, p;
int n, eh;
K ek;
//获得hash
int h = spread(key.hashCode());
//如果表已创建
if ((tab = table) != null && (n = tab.length) > 0 &&
(e = tabAt(tab, (n - 1) & h)) != null) {
if ((eh = e.hash) == h) {
if ((ek = e.key) == key || (ek != null && key.equals(ek)))
return e.val;
}
else if (eh < 0)
return (p = e.find(h, key)) != null ? p.val : null;
while ((e = e.next) != null) {
if (e.hash == h &&
((ek = e.key) == key || (ek != null && key.equals(ek))))
return e.val;
}
}
return null;
}
//保证读取可见性
@SuppressWarnings("unchecked")
static final Node tabAt(Node[] tab, int i) {
return (Node)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
}
public V put(K key, V value) {
return putVal(key, value, false);
}
/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
//不允许key，value为空
if (key == null || value == null)
throw new NullPointerException();
//得到hash
int hash = spread(key.hashCode());
int binCount = 0;
//死循环，直到成功才退出
for (Node[] tab = table;;) {
Node f;
int n, i, fh;
//table 是懒初始化的，没初始化就初始化一遍
if (tab == null || (n = tab.length) == 0)
tab = initTable();
// 计算出table中对象的位置i，获取对象f，找不到
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
//CAS赋值到table[i]
if (casTabAt(tab, i, null,
new Node(hash, key, value, null)))
break; // no lock when adding to empty bin
}// 对象
// MOVED: 前置节点的hash
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
//在node上加锁，就是对应一个hash 加锁
synchronized (f) {
if (tabAt(tab, i) == f) {
/*
MOVED = -1; // hash for forwarding nodes
TREEBIN = -2; // hash for roots of trees
RESERVED = -3; // hash for transient reservations
HASH_BITS = 0x7fffffff; // usable bits of normal node hash
*/
// 链表节点
if (fh >= 0) {
//省略...
}//树节点
else if (f instanceof TreeBin) {
//省略...
}
}
}
}
}
addCount(1L, binCount);
return null;
}
//CAS操作
static final boolean casTabAt(Node[] tab, int i,
Node c, Node v) {
return U.compareAndSwapObject(tab, ((long)i << ASHIFT) + ABASE, c, v);
}
方法总结：
- put方法在插入时，对一个节点加锁，锁的粒度是节点
- put方法调用 casTabAt，对应compareAndSwapObject：CAS保证原子操作
- get方法调用tabAt,对应getObjectVolatile,volatile来保证可见性
- clear 方法调用setTabAt 将table[i]置为空，原子操作
- 尽量少，短的syn同步代码块保证
锁定是短暂的
对比
JDK1.7 和JDK1.8 在ConcurrentHashMaps上的对比：
- 实现原理：
- 1.7
- 是基于段(Segment)实现并发的，并发受到段数量限制
- Segment 继承ReentrantLock来实现锁
- 1.8
- 基于UNSAFE提供的原子操作CAS，volitile
- 使用synchronized实现锁[1] ，
- 1.7
-
锁的粒度：
- 1.7: 锁在Segment 上，相对1.8比较粗糙
- 1.8: 锁在Node上，对应一个hash，更加精细
-
复杂程度：
- 1.7: 简单，代码量1600+
- 1.8: 复杂，代码量6300+
-
退化问题：
- 1.7
- 同一hash以链表解决冲突，
- 过多equals不同而hash相同的对象时，查找退化为二维数组查找
- 节点上查找时间复杂度O(n)，
- 1.8
- 同一个hash上链表长度过大（默认>8）时,会转为红黑树,
- 红黑树查找时间复杂度在O(log(n))级别
- 1.7
synchronized和ReentrantLock 比较
- 对象：
- synchronized 单锁，一次锁一个，嵌套锁来实现复合锁
- ReentrantLock 支持同时锁住多个对象
- 公平：
- synchronized: 不公平
- ReentrantLock
- 支持公平锁，默认不公平
- 按照加锁的时间
- 级别
- synchronized 是关键字
- ReentrantLock 是类级别，提供了诸如{查询请求锁的次数}等操作，更加灵活
- 效率
- synchronized JDK1.7 之前比ReentrantLock 差，1.7或之后效率明显提高
- 大多数情况下，应当优先考虑synchronized
final Lock lock = new ReentrantLock(true);
final Condition notFull = lock.newCondition();
final Condition notEmpty = lock.newCondition();
-
JDK1.8对 synchronized的优化已经等于或优于ReentrantLock了 ↩