如题，实例锁只有一把，而synchronized方法会获得实例对象的锁，下面程序是读写锁的线程，我不明白为什么实例锁只有一把，而读写锁可以多线程同时调用读方法，假设读者线程1调用Data中的read方法，进而调用了readLock方法，获得ReadWriteLock的实例锁，那么读者线程2为什么也可以调用这个synchronized限定的readlock方法，此时的锁不是给了线程1吗？线程2没有锁怎么去调用readlock方法呢？
望大神解答，感谢！！！
- Java code
public final class ReadWriteLock { private int readingReaders = 0; // (A)...实际正在读取的执行绪数量 private int waitingWriters = 0; // (B)...正在等待写入的执行绪数量 private int writingWriters = 0; // (C)...实际正在写入的执行绪数量 private boolean preferWriter = true; // 写入优先的话，值为true public synchronized void readLock() throws InterruptedException { while (writingWriters > 0 || (preferWriter && waitingWriters > 0)) { wait(); } readingReaders++; // (A)实际正在读取的线程数量加1 } public synchronized void readUnlock() { readingReaders--; // (A)实际正在读取的线程数量减1 preferWriter = true; notifyAll(); } public synchronized void writeLock() throws InterruptedException { waitingWriters++; // (B)正在等待写入的线程数量加1 try { while (readingReaders > 0 || writingWriters > 0) { wait(); } } finally {1 waitingWriters--; // (B)正在等待写入的线程数量减1 } writingWriters++; // (C)实际正在写入的线程数量加1 } public synchronized void writeUnlock() { writingWriters--; // (C)实际正在写入的线程数量减 preferWriter = false; notifyAll(); }}// 数据类public class Data { private final char[] buffer; private final ReadWriteLock lock = new ReadWriteLock(); public Data(int size) { this.buffer = new char[size]; for (int i = 0; i < buffer.length; i++) { buffer[i] = '*'; } } public char[] read() throws InterruptedException { lock.readLock(); try { return doRead(); } finally { lock.readUnlock(); } } public void write(char c) throws InterruptedException { lock.writeLock(); try { doWrite(c); } finally { lock.writeUnlock(); } } private char[] doRead() { char[] newbuf = new char[buffer.length]; for (int i = 0; i < buffer.length; i++) { newbuf[i] = buffer[i]; } slowly(); return newbuf; } private void doWrite(char c) { for (int i = 0; i < buffer.length; i++) { buffer[i] = c; slowly(); } } private void slowly() { try { Thread.sleep(50); } catch (InterruptedException e) { } }}------解决方案--------------------------------------------------------
A ReadWriteLock maintains a pair of associated locks, one for read-only operations and one for writing. The read lock may be held simultaneously by multiple reader threads, so long as there are no writers. The write lock is exclusive.
读锁是不互斥的，只要没有写入者。
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跟wait没有关系，读锁和写锁都是逻辑上的锁，物理锁只有一把就是ReadWriteLock实例的锁，而被读取对象数据Data本身并没有加锁；读读不互斥的实现就是在一个线程readlock之后unreadlock之前释放锁时，另一个读者线程取得实例锁，在取得物理锁这一点上是互斥的，但是在读取数据data时却是不互斥的