我读过java文档：ReentrantReadWriteLock
而且我没有看到 writeLock 比 readLock 有任何优先级
但我也读过这样的主题：ReentrantReadWriteLock 中的读写锁是否有某种关联？
我在两个答案中都看到了以下短语：
如果锁被读者持有并且线程请求写锁，则不允许更多的读者获得读锁，直到获得写锁的线程释放它。
喜欢作家胜过读者。也就是说，如果写入者正在等待锁，则不允许来自其他线程的新读取者访问该资源。现有读者可以继续使用该资源，直到他们释放锁。这可以防止所谓的“作家饥饿”。
这些短语听起来很有意义，看起来我已经在其他地方读过。
但显然它与java doc是矛盾的。
它在最新的 JDK 中是否发生了变化？它仍然有效吗？
如何防止作家饿死？
我创建了样本来检查：
public class RWLockTest {
public static final Logger LOGGER = LoggerFactory.getLogger(RWLockTest.class);
public static void main(String[] args) {
SomeClass someClass = new SomeClass();
Reader readerRunnable = new Reader(someClass);
Writer writerRunnable = new Writer(someClass);
//group 1 readers
for (int i = 0; i < 10; i++) {
new Thread(readerRunnable).start();
}
// 2 writers
new Thread(writerRunnable).start();
LOGGER.info("!!!!!!!!!!!!!!!WRITER_1 WAS STARTED!!!!!!!!!!!!!!!");
new Thread(writerRunnable).start();
LOGGER.info("!!!!!!!!!!!!!!!WRITER_2 WAS STARTED!!!!!!!!!!!!!!!");
//group 2 readers
for (int i = 0; i < 10; i++) {
Thread thread = new Thread(readerRunnable);
LOGGER.info(String.format("%s was submitted", thread.getId()));
thread.start();
}
}
public static class SomeClass {
public ReadWriteLock readWriteLock = new ReentrantReadWriteLock();
public void read() {
readWriteLock.readLock().lock();
try {
LOGGER.info(String.format("Read by %s started", Thread.currentThread().getId()));
Thread.sleep(5000);
LOGGER.info(String.format("Read by %s finished", Thread.currentThread().getId()));
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
readWriteLock.readLock().unlock();
}
}
public void write() {
readWriteLock.writeLock().lock();
try {
LOGGER.info(String.format("!!!!!!!!!!Write by %s started!!!!!!!!!!!!", Thread.currentThread().getId()));
Thread.sleep(3000);
LOGGER.info(String.format("!!!!!!!!!!Write by %s finished!!!!!!!!", Thread.currentThread().getId()));
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
readWriteLock.writeLock().unlock();
}
}
}
public static class Reader implements Runnable {
SomeClass someClass;
public Reader(SomeClass someClass) {
this.someClass = someClass;
}
@Override
public void run() {
someClass.read();
}
}
public static class Writer implements Runnable {
SomeClass someClass;
public Writer(SomeClass someClass) {
this.someClass = someClass;
}
@Override
public void run() {
someClass.write();
}
}
}
我尝试运行很多时间，输出通常是这样的：
16:31:49.037 [main] INFO my.pack.RWLockTest - !!!!!!!!!!!!!!!WRITER_1 WAS STARTED!!!!!!!!!!!!!!!
16:31:49.040 [main] INFO my.pack.RWLockTest - !!!!!!!!!!!!!!!WRITER_2 WAS STARTED!!!!!!!!!!!!!!!
16:31:49.046 [Thread-1] INFO my.pack.RWLockTest - Read by 13 started
16:31:49.046 [main] INFO my.pack.RWLockTest - 24 was submitted
16:31:49.046 [Thread-7] INFO my.pack.RWLockTest - Read by 19 started
16:31:49.046 [Thread-4] INFO my.pack.RWLockTest - Read by 16 started
16:31:49.046 [Thread-5] INFO my.pack.RWLockTest - Read by 17 started
16:31:49.046 [Thread-0] INFO my.pack.RWLockTest - Read by 12 started
16:31:49.046 [Thread-3] INFO my.pack.RWLockTest - Read by 15 started
16:31:49.047 [main] INFO my.pack.RWLockTest - 25 was submitted
16:31:49.046 [Thread-9] INFO my.pack.RWLockTest - Read by 21 started
16:31:49.047 [Thread-8] INFO my.pack.RWLockTest - Read by 20 started
16:31:49.047 [main] INFO my.pack.RWLockTest - 26 was submitted
16:31:49.047 [Thread-2] INFO my.pack.RWLockTest - Read by 14 started
16:31:49.047 [Thread-6] INFO my.pack.RWLockTest - Read by 18 started
16:31:49.047 [main] INFO my.pack.RWLockTest - 27 was submitted
16:31:49.048 [main] INFO my.pack.RWLockTest - 28 was submitted
16:31:49.048 [main] INFO my.pack.RWLockTest - 29 was submitted
16:31:49.048 [main] INFO my.pack.RWLockTest - 30 was submitted
16:31:49.048 [main] INFO my.pack.RWLockTest - 31 was submitted
16:31:49.049 [main] INFO my.pack.RWLockTest - 32 was submitted
16:31:49.049 [main] INFO my.pack.RWLockTest - 33 was submitted
16:31:54.047 [Thread-7] INFO my.pack.RWLockTest - Read by 19 finished
16:31:54.048 [Thread-6] INFO my.pack.RWLockTest - Read by 18 finished
16:31:54.047 [Thread-5] INFO my.pack.RWLockTest - Read by 17 finished
16:31:54.049 [Thread-2] INFO my.pack.RWLockTest - Read by 14 finished
16:31:54.051 [Thread-8] INFO my.pack.RWLockTest - Read by 20 finished
16:31:54.047 [Thread-1] INFO my.pack.RWLockTest - Read by 13 finished
16:31:54.050 [Thread-9] INFO my.pack.RWLockTest - Read by 21 finished
16:31:54.049 [Thread-4] INFO my.pack.RWLockTest - Read by 16 finished
16:31:54.049 [Thread-3] INFO my.pack.RWLockTest - Read by 15 finished
16:31:54.049 [Thread-0] INFO my.pack.RWLockTest - Read by 12 finished
16:31:54.057 [Thread-10] INFO my.pack.RWLockTest - !!!!!!!!!!Write by 22 started!!!!!!!!!!!!
16:31:57.057 [Thread-10] INFO my.pack.RWLockTest - !!!!!!!!!!Write by 22 finished!!!!!!!!
16:31:57.058 [Thread-11] INFO my.pack.RWLockTest - !!!!!!!!!!Write by 23 started!!!!!!!!!!!!
16:32:00.060 [Thread-11] INFO my.pack.RWLockTest - !!!!!!!!!!Write by 23 finished!!!!!!!!
16:32:00.061 [Thread-13] INFO my.pack.RWLockTest - Read by 25 started
16:32:00.061 [Thread-14] INFO my.pack.RWLockTest - Read by 26 started
16:32:00.061 [Thread-12] INFO my.pack.RWLockTest - Read by 24 started
16:32:00.061 [Thread-15] INFO my.pack.RWLockTest - Read by 27 started
16:32:00.061 [Thread-17] INFO my.pack.RWLockTest - Read by 29 started
16:32:00.062 [Thread-19] INFO my.pack.RWLockTest - Read by 31 started
16:32:00.062 [Thread-18] INFO my.pack.RWLockTest - Read by 30 started
16:32:00.061 [Thread-16] INFO my.pack.RWLockTest - Read by 28 started
16:32:00.062 [Thread-20] INFO my.pack.RWLockTest - Read by 32 started
16:32:00.062 [Thread-21] INFO my.pack.RWLockTest - Read by 33 started
16:32:05.060 [Thread-12] INFO my.pack.RWLockTest - Read by 24 finished
16:32:05.060 [Thread-15] INFO my.pack.RWLockTest - Read by 27 finished
16:32:05.060 [Thread-13] INFO my.pack.RWLockTest - Read by 25 finished
16:32:05.060 [Thread-17] INFO my.pack.RWLockTest - Read by 29 finished
16:32:05.060 [Thread-14] INFO my.pack.RWLockTest - Read by 26 finished
16:32:05.062 [Thread-21] INFO my.pack.RWLockTest - Read by 33 finished
16:32:05.062 [Thread-16] INFO my.pack.RWLockTest - Read by 28 finished
16:32:05.062 [Thread-19] INFO my.pack.RWLockTest - Read by 31 finished
16:32:05.062 [Thread-18] INFO my.pack.RWLockTest - Read by 30 finished
16:32:05.062 [Thread-20] INFO my.pack.RWLockTest - Read by 32 finished
这是什么意思？
如您所见，我执行以下操作：
如您所见，第一次写入提交（16:31:49.037 [main] INFO my.pack.RWLockTest - !!!!!!!!!!!!!!!WRITER_1 WAS STARTED!!!!!!!!!!!!!!!）和实际声明（（获取的锁）16:31:54.057 [Thread-10] INFO my.pack.RWLockTest - !!!!!!!!!!Write by 22 started!!!!!!!!!!!!）之间的延迟为 5 秒。这是一个阅读的时间。
您还可以看到，在提交了 2 个写入线程后，我们提交了 10 个 id从 24 到 33的读取线程，并且它们实际上都是在写入线程完成工作后启动（获得锁）的。
因此，ID为 24 到 33的读者线程被提交，16:31:49此时前 10 个读者获得了 readlock，看起来他们也能够获得 readlock，但看起来里面有一些东西ReentrantReadWriteLock可以防止它避免 writer 饿死。最终，第二组读者只能在16:32:00（提交后 6 秒）获得锁
我不知道它是否得到保证，但从我的测试来看，它总是以这种方式工作。因此，尽管java doc说：
此类不会为锁访问强加读取器或写入器的优先顺序
ReentrantReadWriteLock 描述了两种操作模式：公平模式和非公平模式。您引用的选择似乎旨在描述公平模式。我不会说两者显然是矛盾的。但是，我可以看到第一个中的不精确措辞（“不允许更多读者”）可能会导致混淆。我怀疑“更多读者”是指来自其他线程的新读者，而不是来自同一线程的额外重入阅读，有些人可能会将其解释为同一个读者。如果以这种方式解释，那么它们似乎与 JavaDoc 一致。
确定的答案总是在代码中，所以让我们看看那里。
这是构造函数（注意：默认构造函数调用这个fair设置为false）
public ReentrantReadWriteLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
readerLock = new ReadLock(this);
writerLock = new WriteLock(this);
}
因此，唯一的区别是sync属性是包含一个FairSync实例还是一个实例NonfairSync。这些实现有何不同？
以下是该类writerShouldBlock方法的代码FairSync：
final boolean writerShouldBlock() {
return hasQueuedPredecessors();
}
这意味着“如果有一条线”，那么作者会阻塞并进入该线（队列）。但是，这与以下类的实现形成鲜明对比NonfairSync：
final boolean writerShouldBlock() {
return false;
}
这明确地表明了non fair mode作家如何获得高于读者的优先权。
关于作家饥饿的最后一条评论。在non fair mode此实现伴随方法的实现：readerShouldBlock. 类中代码的注释NonfairSync指出：
final boolean readerShouldBlock() {
/* As a heuristic to avoid indefinite writer starvation,
* block if the thread that momentarily appears to be head
* of queue, if one exists, is a waiting writer. This is
* only a probabilistic effect since a new reader will not
* block if there is a waiting writer behind other enabled
* readers that have not yet drained from the queue.
*/