面试高频ThreadLocal
ThreadLocal底层数据结构
相关的CSDN博客：
ThreadLocal ，也叫线程本地变量，ThreadLocal为变量在每个线程中都创建了所使用的的变量副本。使用起来都是在线程的本地工作内存中操作，并且提供了set和get方法来访问拷贝过来的变量副本。底层也是封装了ThreadLocalMap集合类来绑定当前线程和变量副本的关系，各个线程独立并且访问安全！
底层：
其实ThreadLocal里面封装了ThreadLocalMap集合类来绑定当前线程和变量副本的关系。
ThreadLocalMap其实就是利用数组进行实现的。跟HashMap相似
根据key.threadLocalHashCode & (table.length - 1);获取下标值，然后获取到数组的值
static class ThreadLocalMap {
static class Entry extends WeakReference<ThreadLocal> {
Object value;
Entry(ThreadLocal k, Object v) {
super(k);
value = v;
}
}
// 初始容量
private static final int INITIAL_CAPACITY = 16;
// 核心数组
private Entry[] table;
private int size = 0;
private int threshold; // Default to 0
private void setThreshold(int len) {
threshold = len * 2 / 3;
}
// i的下一个下标，其实就是保证循环
private static int nextIndex(int i, int len) {
return ((i + 1 < len) ? i + 1 : 0);
}
// i的上一个下标，其实就是保证循环
private static int prevIndex(int i, int len) {
return ((i - 1 >= 0) ? i - 1 : len - 1);
}
ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
private ThreadLocalMap(ThreadLocalMap parentMap) {
Entry[] parentTable = parentMap.table;
int len = parentTable.length;
setThreshold(len);
table = new Entry[len];
for (int j = 0; j < len; j++) {
Entry e = parentTable[j];
if (e != null) {
ThreadLocal key = e.get();
if (key != null) {
Object value = key.childValue(e.value);
Entry c = new Entry(key, value);
int h = key.threadLocalHashCode & (len - 1);
while (table[h] != null)
h = nextIndex(h, len);
table[h] = c;
size++;
}
}
}
}
/**
* 根据key获取到Entry的值
*/
private Entry getEntry(ThreadLocal key) {
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[i];
if (e != null && e.get() == key)
return e;
else
return getEntryAfterMiss(key, i, e);
}
/**
* 当key没有在hash槽中出现的时候，需要根据这个方法进行获取
*/
private Entry getEntryAfterMiss(ThreadLocal key, int i, Entry e) {
Entry[] tab = table;
int len = tab.length;
while (e != null) {
ThreadLocal k = e.get();
if (k == key)
return e;
if (k == null)
expungeStaleEntry(i);
else
i = nextIndex(i, len);
e = tab[i];
}
return null;
}
/**
* 设置key的值为value
*/
private void set(ThreadLocal key, Object value) {
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];e != null;e = tab[i = nextIndex(i, len)]) {
ThreadLocal k = e.get();
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
/**
* 移除key
*/
private void remove(ThreadLocal key) {
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];e != null;e = tab[i = nextIndex(i, len)]) {
if (e.get() == key) {
e.clear();
expungeStaleEntry(i);
return;
}
}
}
/**
* Replace a stale entry encountered during a set operation
* with an entry for the specified key. The value passed in
* the value parameter is stored in the entry, whether or not
* an entry already exists for the specified key.
*
* As a side effect, this method expunges all stale entries in the
* "run" containing the stale entry. (A run is a sequence of entries
* between two null slots.)
*/
private void replaceStaleEntry(ThreadLocal key, Object value,
int staleSlot) {
Entry[] tab = table;
int len = tab.length;
Entry e;
// Back up to check for prior stale entry in current run.
// We clean out whole runs at a time to avoid continual
// incremental rehashing due to garbage collector freeing
// up refs in bunches (i.e., whenever the collector runs).
int slotToExpunge = staleSlot;
for (int i = prevIndex(staleSlot, len);
(e = tab[i]) != null;
i = prevIndex(i, len))
if (e.get() == null)
slotToExpunge = i;
// Find either the key or trailing null slot of run, whichever
// occurs first
for (int i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal k = e.get();
// If we find key, then we need to swap it
// with the stale entry to maintain hash table order.
// The newly stale slot, or any other stale slot
// encountered above it, can then be sent to expungeStaleEntry
// to remove or rehash all of the other entries in run.
if (k == key) {
e.value = value;
tab[i] = tab[staleSlot];
tab[staleSlot] = e;
// Start expunge at preceding stale entry if it exists
if (slotToExpunge == staleSlot)
slotToExpunge = i;
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
return;
}
// If we didn't find stale entry on backward scan, the
// first stale entry seen while scanning for key is the
// first still present in the run.
if (k == null && slotToExpunge == staleSlot)
slotToExpunge = i;
}
// If key not found, put new entry in stale slot
tab[staleSlot].value = null;
tab[staleSlot] = new Entry(key, value);
// If there are any other stale entries in run, expunge them
if (slotToExpunge != staleSlot)
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
}
/**
* Expunge a stale entry by rehashing any possibly colliding entries
* lying between staleSlot and the next null slot. This also expunges
* any other stale entries encountered before the trailing null. See
* Knuth, Section 6.4
*
* @param staleSlot index of slot known to have null key
* @return the index of the next null slot after staleSlot
* (all between staleSlot and this slot will have been checked
* for expunging).
*/
private int expungeStaleEntry(int staleSlot) {
Entry[] tab = table;
int len = tab.length;
// expunge entry at staleSlot
tab[staleSlot].value = null;
tab[staleSlot] = null;
size--;
// Rehash until we encounter null
Entry e;
int i;
for (i = nextIndex(staleSlot, len);(e = tab[i]) != null;i = nextIndex(i, len)) {
ThreadLocal k = e.get();
if (k == null) {
e.value = null;
tab[i] = null;
size--;
} else {
int h = k.threadLocalHashCode & (len - 1);
if (h != i) {
tab[i] = null;
while (tab[h] != null)
h = nextIndex(h, len);
tab[h] = e;
}
}
}
return i;
}
/**
* Re-pack and/or re-size the table. First scan the entire
* table removing stale entries. If this doesn't sufficiently
* shrink the size of the table, double the table size.
*/
private void rehash() {
expungeStaleEntries();
// Use lower threshold for doubling to avoid hysteresis
if (size >= threshold - threshold / 4)
resize();
}
/**
* Double the capacity of the table.
*/
private void resize() {
Entry[] oldTab = table;
int oldLen = oldTab.length;
int newLen = oldLen * 2;
Entry[] newTab = new Entry[newLen];
int count = 0;
for (int j = 0; j < oldLen; ++j) {
Entry e = oldTab[j];
if (e != null) {
ThreadLocal k = e.get();
if (k == null) {
e.value = null; // Help the GC
} else {
int h = k.threadLocalHashCode & (newLen - 1);
while (newTab[h] != null)
h = nextIndex(h, newLen);
newTab[h] = e;
count++;
}
}
}
setThreshold(newLen);
size = count;
table = newTab;
}
} 1、set方法
public void set(T value) {
Thread t = Thread.currentThread();//1.首先获取当前线程对象
ThreadLocalMap map = getMap(t);//2.获取该线程对象的ThreadLocalMap
if (map != null)
map.set(this, value);//如果map不为空，执行set操作，以当前threadLocal对象为key，实际存储对象为value进行set操作
else
createMap(t, value);//如果map为空，则为该线程创建ThreadLocalMap
} ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
//threadLocals是在Thread类中定义的变量
ThreadLocal.ThreadLocalMap threadLocals = null;
// 若该线程没有ThreadLocalMap对象，需要进行创建
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
2、get方法
获取到当前线程的ThreadLocalMap对象，然后获取到该对象的值
如果没有的话，就创建该ThreadLocalMap
public T get() {
Thread t = Thread.currentThread(); // 获取到当前线程
ThreadLocalMap map = getMap(t); // 获取到当前线程的ThreadLocalMap对象
// 如果不为空就尝试获取值，为空就调用setInitialValue来进行创建初始值的ThreadLocalMap对象
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null)
return (T)e.value;
}
return setInitialValue();
}
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
} 4、ReentrantLock的底层
ReentrantLock的源码：
（1）ReentrantLock是Lock的子类，支持序列化
（2）ReentrantLock里面有一抽象的静态内部类Sync（继承了AQS），并且有一类型为该类Sync的成员变量
有两个Sync的子类，表示公平锁和非公平锁
（3）ReentrantLock里面的方法都是通过sync调用其里面的方法进行实现的
（4）ReentrantLock里面是默认使用非公平锁的，如果要使用公平锁，则在lock定义的时候需要传入参数true
state初始化为0，表示未锁定状态。A线程lock时，会调用tryAcquire独占该锁并将state+acquires（一般acquires就是1）
public class ReentrantLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = 7373984872572414699L;
private final Sync sync;
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -5179523762034025860L;
/**
* 加锁
*/
abstract void lock();
/**
* 非公平锁尝试获取锁
*/
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
// 0 表示未锁定状态
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
/**
* 尝试释放资源，成功则返回true，失败则返回false。
*/
protected final boolean tryRelease(int releases) {
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c);
return free;
}
/**
* 该线程是否正在独占资源。只有用到condition才需要去实现它
*/
protected final boolean isHeldExclusively() {
return getExclusiveOwnerThread() == Thread.currentThread();
}
final ConditionObject newCondition() {
return new ConditionObject();
}
final Thread getOwner() {
return getState() == 0 ? null : getExclusiveOwnerThread();
}
final int getHoldCount() {
return isHeldExclusively() ? getState() : 0;
}
final boolean isLocked() {
return getState() != 0;
}
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
setState(0); // reset to unlocked state
}
}
public ReentrantLock() {
sync = new NonfairSync();
}
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
public void lock() {
sync.lock();
}
public void lockInterruptibly() throws InterruptedException {
sync.acquireInterruptibly(1);
}
public boolean tryLock() {
return sync.nonfairTryAcquire(1);
}
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
}
public void unlock() {
sync.release(1);
}
public Condition newCondition() {
return sync.newCondition();
}
public int getHoldCount() {
return sync.getHoldCount();
}
public boolean isHeldByCurrentThread() {
return sync.isHeldExclusively();
}
public boolean isLocked() {
return sync.isLocked();
}
public final boolean isFair() {
return sync instanceof FairSync;
}
protected Thread getOwner() {
return sync.getOwner();
}
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
public final boolean hasQueuedThread(Thread thread) {
return sync.isQueued(thread);
}
public final int getQueueLength() {
return sync.getQueueLength();
}
protected Collection<Thread> getQueuedThreads() {
return sync.getQueuedThreads();
}
public boolean hasWaiters(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
}
public int getWaitQueueLength(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
}
protected Collection<Thread> getWaitingThreads(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
}
public String toString() {
Thread o = sync.getOwner();
return super.toString() + ((o == null) ?
"[Unlocked]" :
"[Locked by thread " + o.getName() + "]");
}
} 重入锁与非重入锁
ReentrantLock是一个可重入锁，尝试获取锁tryAcquire方法是不会引起阻塞的，lock方法是会引起阻塞的
先列举出其源码：
重入锁尝试获取锁的源码如下：
/**
* 尝试获取资源，成功则返回true，失败则返回false。
*/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
//首先判断当前是否已经有线程获取到锁 ， 0表示没有获取到锁
if (c == 0) {
if (!hasQueuedPredecessors() && compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current); // 设置当前线程为独占该锁的线程
return true;
}
// 若已经有线程获取到锁了，则判断这个线程是不是就是当前的线程（可重入的原因）
}else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
} - 可重入锁尝试获取资源的方法逻辑是，首先判断当前是否已经有线程获取到锁
- 若当前锁没有被占用，则当前线程可以进行占用到这个锁，变为独占锁
- 若当前锁被占用了，这个时候要比较这个占用的锁的线程是不是当前线程，如果是，也相当于获取到锁的。着也是与非重入锁差别的一点
若改为非重入锁，尝试获取锁的源码如下：
/**
* 尝试获取资源，成功则返回true，失败则返回false。
*/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
return false;
} - 不可重入锁尝试获取资源的方法逻辑是，首先判断当前是否已经有线程获取到锁
- 若当前锁没有被占用，则当前线程可以进行占用到这个锁，变为独占锁
- 若当前锁被占用了，这个时候就直接返回获取锁失败false（它不会去判断占用这个锁的线程是不是就是当前线程）
公平锁与非公平锁
ReentrantLock里面是默认使用非公平锁的，如果要使用公平锁，则在lock定义的时候需要传入参数true
state初始化为0，表示未锁定状态。A线程lock时，会调用tryAcquire独占该锁并将state+acquires
（1）公平锁与非公平锁的区别：
体现在获取锁（调用lock方法）的时候，非公平锁是先进行CAS操作，进行判断是否可以直接获取到锁，则公平锁就直接进行获取锁的操作
// 非公平锁
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
/**
* 先进行CAS判断，是否可以获取到锁，如果可以获取到锁，就将当前独占的锁设置为当前进程
*/
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
// nonfairTryAcquire查看上面Sync中的
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}
// 公平锁
static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L;
/**
* state 为 0 表示未锁定状态，加锁，就给state加1
*/
final void lock() {
acquire(1);
}
/**
* 尝试获取资源，成功则返回true，失败则返回false。
*/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
}
// 获取到锁
public final void acquire(int arg) {
if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
} AQS中的CAS操作，通过预期值与内存的值进行比较，若相同，则进行更新
protected final boolean compareAndSetState(int expect, int update) {
// unsafe一般用于原子Atmoic类中，底层相关的东西
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
} #Java##学习路径#