大家或多或少会接触一些线程安全问题,什么是线程安全?
通俗的来讲,某个函数被多个线程调用多次,都能够处理各个线程中的局部变量,并且计算结果正确,我们一般称为线程安全。
如何解决线程安全问题?
一般有三种方式
Lock 的实现原理是什么?
AQS(AbstracctQueuedSynchronized) 队列同步器,是用来构建锁或者其他同步器组件的基础框架。
AQS 使用了一个 int 变量来表示同步状态,通过内置的 FIFO 队列来完成资源获取线程的排队工作。
经常使用的同步组件ReentrantLock、ReentrantReadWriteLock和 CountDownLatch 等都是基于同步器实现的。
AQS 主要包含两点,一个是同步状态,第二个是队列。
AQS 是怎么实现线程同步的?主要包括同步队列、独占是同步状态的释放和获取、共享式同步状态的释放和获取。
同步器依赖的是同步队列的来进行同步状态的管理。
同步队列的结构
队列中的节点 Node 是构成同步器的基础。
static final class 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;
/**
* Status field, taking on only the values:
* SIGNAL: The successor of this node is (or will soon be)
* blocked (via park), so the current node must
* unpark its successor when it releases or
* cancels. To avoid races, acquire methods must
* first indicate they need a signal,
* then retry the atomic acquire, and then,
* on failure, block.
* CANCELLED: This node is cancelled due to timeout or interrupt.
* Nodes never leave this state. In particular,
* a thread with cancelled node never again blocks.
* CONDITION: This node is currently on a condition queue.
* It will not be used as a sync queue node
* until transferred, at which time the status
* will be set to 0. (Use of this value here has
* nothing to do with the other uses of the
* field, but simplifies mechanics.)
* PROPAGATE: A releaseShared should be propagated to other
* nodes. This is set (for head node only) in
* doReleaseShared to ensure propagation
* continues, even if other operations have
* since intervened.
* 0: None of the above
*
* The values are arranged numerically to simplify use.
* Non-negative values mean that a node doesn't need to
* signal. So, most code doesn't need to check for particular
* values, just for sign.
*
* The field is initialized to 0 for normal sync nodes, and
* CONDITION for condition nodes. It is modified using CAS
* (or when possible, unconditional volatile writes).
*/
volatile int waitStatus;
/**
* Link to predecessor node that current node/thread relies on
* for checking waitStatus. Assigned during enqueuing, and nulled
* out (for sake of GC) only upon dequeuing. Also, upon
* cancellation of a predecessor, we short-circuit while
* finding a non-cancelled one, which will always exist
* because the head node is never cancelled: A node becomes
* head only as a result of successful acquire. A
* cancelled thread never succeeds in acquiring, and a thread only
* cancels itself, not any other node.
*/
volatile Node prev;
/**
* Link to the successor node that the current node/thread
* unparks upon release. Assigned during enqueuing, adjusted
* when bypassing cancelled predecessors, and nulled out (for
* sake of GC) when dequeued. The enq operation does not
* assign next field of a predecessor until after attachment,
* so seeing a null next field does not necessarily mean that
* node is at end of queue. However, if a next field appears
* to be null, we can scan prev's from the tail to
* double-check. The next field of cancelled nodes is set to
* point to the node itself instead of null, to make life
* easier for isOnSyncQueue.
*/
volatile Node next;
/**
* The thread that enqueued this node. Initialized on
* construction and nulled out after use.
*/
volatile Thread thread;
/**
* Link to next node waiting on condition, or the special
* value SHARED. Because condition queues are accessed only
* when holding in exclusive mode, we just need a simple
* linked queue to hold nodes while they are waiting on
* conditions. They are then transferred to the queue to
* re-acquire. And because conditions can only be exclusive,
* we save a field by using special value to indicate shared
* mode.
*/
Node nextWaiter;
/**
* Returns true if node is waiting in shared mode.
*/
final boolean isShared() {
return nextWaiter == SHARED;
}
/**
* Returns previous node, or throws NullPointerException if null.
* Use when predecessor cannot be null. The null check could
* be elided, but is present to help the VM.
*
* @return the predecessor of this node
*/
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null)
throw new NullPointerException();
else
return p;
}
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;
}
}
Node 的构造方法可以看到,包含了线程 Thread 和 状态 waitStatus 或者 Thread 和 nextWaiter(Node) 。
/** 值为1,由于同步队列中等待的线程超时或者被中断,需要到同步队列中取消等待,节点进入该状态将不会变*/
static final int CANCELLED = 1;
/**后继节点的线程处于阻塞状态,而如果当前节点的线程如果释放同步状态或者被取消,通知后继节点,使得后继节点可以运行*/
static final int SIGNAL = -1;
/** 值为-2 节点在等待队列中,节点线程等待在Condition上,当其他线程对 Condition 调用了 signal() 方法后,该节点会从等待队列转移到同步队列中,进行同步状态的获取 */
static final int CONDITION = -2;
节点加入到同步队列
同步器拥有首节点 head 和 尾节点 tail 没有成功获取同步状态的线程将会组成Node 加入该队列的尾部。这个加入队尾的过程需要是线程安全的。同步器提供了一个基于 CAS 的设置尾节点的方法 compareAndSetTail(Node expt, Node update) 需要传递当前线程认为的尾节点 expt 和当前节点 update。
为什么 CAS 能够保证线程安全?
java 中的 CAS 是对 cmpxchg 的封装。
cmpxchg 中x86 中有 CAS 指令。 cmpxchg是汇编指令 作用:比较并交换操作数. 如:CMPXCHG r/m,r 将累加器AL/AX/EAX/RAX中的值与首操作数(目的操作数)比较,如果相等,第2操作数(源操作数)的值装载到首操作数,zf置1。如果不等, 首操作数的值装载到AL/AX/EAX/RAX并将zf清0 该指令只能用于486及其后继机型。第2操作数(源操作数)只能用8位、16位或32位寄存器。第1操作数(目地操作数)则可用寄存器或任一种存储器寻址方式
cmpxchg 功能就是保证一次只原子性的修改一个变量。
线程释放同步状态,节点出队
首节点的线程在释放同步状态时,将会唤醒后继节点。而后继节点将会在获取同步状态时,将自己设置成首节点。
设置首节点是通过获取同步状态成功的线程来完成的,由于只有一个线程能够成功的获取同步状态,因此,不需要使用 CAS 来保证只需要将首节点的后继节点设置成首节点即可。