<br />所有的同步场景都是基于锁。锁在并发编程中发挥重要作用。<br /> <br />我是李福春,我在准备面试,今天的题目是:<br /> <br /> <br />
synchronized底层是如何实现的?
答: synchronized是在底层的jvm中实现的,即c++写的,synchronized的实现是基于一对monitorenter, monitorexit指令实现的,monitor对象是同步的基本实现单元。<br /> <br />在java6中,monitor依靠操作系统提供的内部互斥锁,需要在用户态空间和内核态空间切换,所以同步操作是一个比较重的操作,开销比较大。<br /> <br />在java7之后,monitor有三种不同的实现,即偏斜锁,轻量级锁,重量级锁。<br /> <br />基于对象头的markword,标记上偏向的线程id, 在没有竞争的条件下,使用的是偏斜锁;<br />当有多个线程来竞争偏斜锁,基于cas对对象头的markword来进行竞争,如果拿到了,升级为轻量级锁。<br />没拿到则升级为重量级锁;<br /> <br />锁降级发生在jvm进入安全点检查的时候,对monitor进行降级。<br /> <br />
synchronized底层实现
<br />对象头结构:<br />
<br /> <br /> <br />sharedRuntime.cpp 解释器和编译器的运行时基类。 ```cpp
Handle h_obj(THREAD, obj); if (UseBiasedLocking) { // Retry fast entry if bias is revoked to avoid unnecessary inflation ObjectSynchronizer::fast_enter(h_obj, lock, true, CHECK); } else { ObjectSynchronizer::slow_enter(h_obj, lock, CHECK); }
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<br />偏斜锁逻辑代码:<br />
```cpp
void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock,
bool attempt_rebias, TRAPS) {
if (UseBiasedLocking) {
if (!SafepointSynchronize::is_at_safepoint()) {
BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
return;
}
} else {
assert(!attempt_rebias, "can not rebias toward VM thread");
BiasedLocking::revoke_at_safepoint(obj);
}
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
slow_enter(obj, lock, THREAD);
}
<br /> <br />轻量级锁: ```cpp
void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) { markOop mark = obj->mark(); if (mark->is_neutral()) { // 将目前的Mark Word复制到Displaced Header上 lock->set_displaced_header(mark); // 利用CAS设置对象的Mark Word if (mark == obj()->cas_set_mark((markOop) lock, mark)) { TEVENT(slow_enter: release stacklock); return; } // 检查存在竞争 } else if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) { // 清除 lock->set_displaced_header(NULL); return; }
// 重置Displaced Header lock->set_displaced_header(markOopDesc::unused_mark()); ObjectSynchronizer::inflate(THREAD, obj(), inflate_cause_monitor_enter)->enter(THREAD); }
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# java体系中的锁
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<br />
<br />java提供的锁有哪些<br />
<br />
![image.png](https://oscimg.oschina.net/oscnet/up-6852ef51cdfe1d398b7dc641595a7ef425c.png)
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<br />
<br />再入读写锁:<br />
<br />
```java
public class RWSample {
private final Map<String, String> m = new TreeMap<>();
private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
private final Lock r = rwl.readLock();
private final Lock w = rwl.writeLock();
public String get(String key) {
r.lock();
System.out.println("读锁锁定!");
try {
return m.get(key);
} finally {
r.unlock();
}
}
public String put(String key, String entry) {
w.lock();
System.out.println("写锁锁定!");
try {
return m.put(key, entry);
} finally {
w.unlock();
}
}
// …
}
<br />优化的读写锁:StampedLock<br />代码如下:<br />读取之前先通过validate方法判断是否进入写状态,如果没有进入写状态,不用加读锁,避免了开销;<br />否则增加读锁,保证一致性。<br />
public class StampedSample {
private final StampedLock sl = new StampedLock();
void mutate() {
long stamp = sl.writeLock();
try {
write();
} finally {
sl.unlockWrite(stamp);
}
}
Data access() {
long stamp = sl.tryOptimisticRead();
Data data = read();
if (!sl.validate(stamp)) {
stamp = sl.readLock();
try {
data = read();
} finally {
sl.unlockRead(stamp);
}
}
return data;
}
// …
}
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小结
<br />本篇先介绍了synchronized的底层实现原理,介绍了java8中引入的3种级别的锁,以及锁升级和降级过程。<br /> <br />然后介绍了java体系中的其它的锁和优化读写锁使用场景。<br /> <br /> <br /> <br /> <br />
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来源:oschina
链接:https://my.oschina.net/carterspring/blog/3212534