Java核心复习——J.U.C AbstractQueuedSynchronizer
2024-10-06 08:10:41
背景和作用
在同步组件的实现中，AQS是核心部分，同步组件的实现者通过使用AQS提供的模板方法实现同步组件语义，AQS则实现了对同步状态的管理，以及对阻塞线程进行排队，等待通知等等一些底层的实现处理。AQS的核心也包括了这些方面:同步队列，独占式锁的获取和释放，共享锁的获取和释放以及可中断锁，超时等待锁获取这些特性的实现。
AQS
public abstract class AbstractOwnableSynchronizer
implements java.io.Serializable {
/**
* Empty constructor for use by subclasses.
*/
protected AbstractOwnableSynchronizer() { }
/**
* The current owner of exclusive mode synchronization.
*/
//独占模式下的线程
private transient Thread exclusiveOwnerThread;
// getter、setter
protected final void setExclusiveOwnerThread(Thread thread) {
exclusiveOwnerThread = thread;
}
protected final Thread getExclusiveOwnerThread() {
return exclusiveOwnerThread;
}
}
AQS#Node
/** Marker to indicate a node is waiting in shared mode */
static final Node SHARED = new Node();
/** Marker to indicate a node is waiting in exclusive mode */
static final Node EXCLUSIVE = null;
/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL = -1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = -2;
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
static final int PROPAGATE = -3;
// 该变量主要记录线程的状态，确保线程是有效的，CANCELLED、SIGNAL、CONDITION、PROPAGATE
volatile int waitStatus;
volatile Node prev;
volatile Node next;
volatile Thread thread;
Node() { // Used to establish initial head or SHARED marker
}
Node(Thread thread, Node mode) { // Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
}
Node(Thread thread, int waitStatus) { // Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
AbstractQueuedSynchronizer
AQS使用CLH队列来实现线程排队的功能，CLH队列的原理是一个双向链表，每个链表由Node节点组成，Node节点保存prev和next两个指针，并保存指向线程的指针。
/**
* Head of the wait queue, lazily initialized. Except for
* initialization, it is modified only via method setHead. Note:
* If head exists, its waitStatus is guaranteed not to be
* CANCELLED.
*/
// 记录CLH的头部节点，目的是配合tail实现出队操作
private transient volatile Node head;
/**
* Tail of the wait queue, lazily initialized. Modified only via
* method enq to add new wait node.
*/
// 记录CLH的尾部节点
private transient volatile Node tail;
/**
* The synchronization state.
*/
// 用于记录加锁的次数，同一个线程加两次锁，则state = 2
private volatile int state;
/**
* Inserts node into queue, initializing if necessary. See picture above.
* @param node the node to insert
* @return node's predecessor
*/
// 入队操作
private Node enq(final Node node) {
for (;;) {
Node t = tail;
if (t == null) { // Must initialize
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
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