1 偏向锁的意义
无多线程竞争时，减少不必要的轻量级锁执行路径。大多数情况下，锁不仅不存在多线程竞争，而且总是由同一条线程去多次获得锁，为了让线程获得锁的性能代价更低而引入了偏向锁。
偏向锁主要用来优化同一线程多次申请同一个锁的竞争，即当对象被当做同步锁并有一个线程抢到了锁时，则在Mark Word设置该线程的线程ID、是否偏向锁设置1、锁标志位设置01等信息，此时的Mark Word 存储的就是偏向锁状态信息。
在：
-
创建一个线程并在线程中执行循环监听的场景下
-
或单线程操作一个线程安全集合时
同一线程每次都需获取和释放锁，每次操作都会发生用户态与内核态的切换。
获取偏向锁的场景：
在自己的线程栈生成一条Lock Record，然后Object Reference指向对象头，此时Lock Record与对象头就建立了联系：
① : 先判断Mard Word的Thread ID是否有值
- 没有，则表示当前资源没有被其他线程占用，把当前线程ID等信息记录到Mark Word（这需CAS，可能多条线程修改Mark Word，需要保证原子性）
-
有，则表示当前资源被线程占用，需要判断该线程是不是自己
- 该线程ID是自己的，则表示可重入，直接获取(此时在自己的线程栈中继续生成一条新的Lock Record)
- 该线程ID不是自己的，说明出现其他线程竞争，当前持有偏向锁的线程就需要撤销了，即当其他线程尝试获取偏向锁才释放锁
轻量级锁的获取及释放依赖多次的CAS操作，而偏向锁只依赖一次CAS置换ThreadID。
一旦出现多个线程竞争时必须撤销偏向锁，所以：
撤销偏向锁消耗的性能必须 ＜ 之前节省下来的CAS原子操作的性能消耗
不然得不偿失!
JDK6默认开启偏向锁，可通过-XX:-UseBiasedLocking禁用偏向锁。
2 偏向锁的获取偏向锁的入口，synchronizer.cpp 文件的
ObjectSynchronizer::fast_enter
由BiasedLocking::revoke_and_rebias实现
获取对象的markOop数据mark，即对象头的Mark Word
mark的偏向锁的锁标志位为 01
2.3 判断mark中JavaThread的状态- 若指向当前线程，则执行同步代码块
- 若为空，则走4
- 若指向其它线程，则走5
设置mark中JavaThread为当前线程ID。
若CAS成功，则执行同步代码块，否则走5。
2.5 执行CAS失败说明当前存在多个线程竞争锁，当达到全局安全点（safepoint），获得偏向锁的线程就会被挂起，撤销偏向锁，并升级为轻量级锁。
升级完成后被阻塞在安全点的线程继续执行同步代码块。
BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
// We can revoke the biases of anonymously-biased objects
// efficiently enough that we should not cause these revocations to
// update the heuristics because doing so may cause unwanted bulk
// revocations (which are expensive) to occur.
// step1
markOop mark = obj->mark();
if (mark->is_biased_anonymously() && !attempt_rebias) {
// We are probably trying to revoke the bias of this object due to
// an identity hash code computation. Try to revoke the bias
// without a safepoint. This is possible if we can successfully
// compare-and-exchange an unbiased header into the mark word of
// the object, meaning that no other thread has raced to acquire
// the bias of the object.
markOop biased_value = mark;
markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
if (res_mark == biased_value) {
return BIAS_REVOKED;
}
} else if (mark->has_bias_pattern()) {
Klass* k = obj->klass();
markOop prototype_header = k->prototype_header();
if (!prototype_header->has_bias_pattern()) {
// This object has a stale bias from before the bulk revocation
// for this data type occurred. It's pointless to update the
// heuristics at this point so simply update the header with a
// CAS. If we fail this race, the object's bias has been revoked
// by another thread so we simply return and let the caller deal
// with it.
markOop biased_value = mark;
markOop res_mark = (markOop) Atomic::cmpxchg_ptr(prototype_header, obj->mark_addr(), mark);
assert(!(*(obj->mark_addr()))->has_bias_pattern(), "even if we raced, should still be revoked");
return BIAS_REVOKED;
} else if (prototype_header->bias_epoch() != mark->bias_epoch()) {
// The epoch of this biasing has expired indicating that the
// object is effectively unbiased. Depending on whether we need
// to rebias or revoke the bias of this object we can do it
// efficiently enough with a CAS that we shouldn't update the
// heuristics. This is normally done in the assembly code but we
// can reach this point due to various points in the runtime
// needing to revoke biases.
if (attempt_rebias) {
assert(THREAD->is_Java_thread(), "");
markOop biased_value = mark;
markOop rebiased_prototype = markOopDesc::encode((JavaThread*) THREAD, mark->age(), prototype_header->bias_epoch());
markOop res_mark = (markOop) Atomic::cmpxchg_ptr(rebiased_prototype, obj->mark_addr(), mark);
if (res_mark == biased_value) {
return BIAS_REVOKED_AND_REBIASED;
}
} else {
markOop biased_value = mark;
markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
if (res_mark == biased_value) {
return BIAS_REVOKED;
}
}
}
}
HeuristicsResult heuristics = update_heuristics(obj(), attempt_rebias);
if (heuristics == HR_NOT_BIASED) {
return NOT_BIASED;
} else if (heuristics == HR_SINGLE_REVOKE) {
Klass *k = obj->klass();
markOop prototype_header = k->prototype_header();
if (mark->biased_locker() == THREAD &&
prototype_header->bias_epoch() == mark->bias_epoch()) {
// A thread is trying to revoke the bias of an object biased
// toward it, again likely due to an identity hash code
// computation. We can again avoid a safepoint in this case
// since we are only going to walk our own stack. There are no
// races with revocations occurring in other threads because we
// reach no safepoints in the revocation path.
// Also check the epoch because even if threads match, another thread
// can come in with a CAS to steal the bias of an object that has a
// stale epoch.
ResourceMark rm;
if (TraceBiasedLocking) {
tty->print_cr("Revoking bias by walking my own stack:");
}
EventBiasedLockSelfRevocation event;
BiasedLocking::Condition cond = revoke_bias(obj(), false, false, (JavaThread*) THREAD, NULL);
((JavaThread*) THREAD)->set_cached_monitor_info(NULL);
assert(cond == BIAS_REVOKED, "why not?");
if (event.should_commit()) {
event.set_lockClass(k);
event.commit();
}
return cond;
} else {
EventBiasedLockRevocation event;
VM_RevokeBias revoke(&obj, (JavaThread*) THREAD);
VMThread::execute(&revoke);
if (event.should_commit() && (revoke.status_code() != NOT_BIASED)) {
event.set_lockClass(k);
// Subtract 1 to match the id of events committed inside the safepoint
event.set_safepointId(SafepointSynchronize::safepoint_counter() - 1);
event.set_previousOwner(revoke.biased_locker());
event.commit();
}
return revoke.status_code();
}
}
assert((heuristics == HR_BULK_REVOKE) ||
(heuristics == HR_BULK_REBIAS), "?");
EventBiasedLockClassRevocation event;
VM_BulkRevokeBias bulk_revoke(&obj, (JavaThread*) THREAD,
(heuristics == HR_BULK_REBIAS),
attempt_rebias);
VMThread::execute(&bulk_revoke);
if (event.should_commit()) {
event.set_revokedClass(obj->klass());
event.set_disableBiasing((heuristics != HR_BULK_REBIAS));
// Subtract 1 to match the id of events committed inside the safepoint
event.set_safepointId(SafepointSynchronize::safepoint_counter() - 1);
event.commit();
}
return bulk_revoke.status_code();
}
3 偏向锁的撤销
只有当其它线程尝试竞争偏向锁时，持有偏向锁的线程才会释放锁。
偏向锁的撤销由BiasedLocking::revoke_at_safepoint实现：
void BiasedLocking::revoke_at_safepoint(Handle h_obj) {
assert(SafepointSynchronize::is_at_safepoint(), "must only be called at safepoint");
oop obj = h_obj();
HeuristicsResult heuristics = update_heuristics(obj, false);
if (heuristics == HR_SINGLE_REVOKE) {
revoke_bias(obj, false, false, NULL, NULL);
} else if ((heuristics == HR_BULK_REBIAS) ||
(heuristics == HR_BULK_REVOKE)) {
bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);
}
clean_up_cached_monitor_info();
}
- 偏向锁的撤销动作必须等待全局安全点（safepoint，GC时会让所有线程阻塞的停顿点）
- 暂停拥有偏向锁的线程，判断锁对象是否处于被锁定状态
- 撤销偏向锁，恢复到无锁（标志位 01）或轻量级锁（标志位 00）状态
偏向锁在Java 1.6后默认启用，但在应用程序启动几s后才激活，可关闭延迟：
-XX:BiasedLockingStartupDelay=0
若确定应用程序中所有锁通常情况下处于竞争状态，可关闭偏向锁：
XX:-UseBiasedLocking=false（默认打开）偏向锁的释放
遍历线程栈的所有Lock Record,把ObjectReference切断,即ObjectReference = null.
把ObjectReference置null，但锁对象的对象头的Mark Word还是没改变，依然偏向之前的线程，那还是没释放锁的嘛，的确是，线程退出临界区时候，并没有释放偏向锁，这么做是为 : 当再次需要获取锁时，只需要简单判断是否是重入，即可快速获取锁，而不用每次都CAS，这也是偏向锁在只有一个线程访问锁的情景下高效的核心。
总结- 当出现锁资源访问的时候，都会在当前线程栈生成一条Lock Record，并且ObjectReference将指向锁对象的对象头 的Mark Word，该设置可能出现多线程，需CAS操作
- 多线程情况下竞争同一个锁资源，偏向锁的撤销会影响效率
- 偏向锁的重入计数依靠线程栈里Lock Record个数
- 偏向锁撤销失败，最终会升级为轻量级锁
- 偏向锁退出时并没有修改Mark Word，也就是没有释放锁
- 偏向锁相对轻量级锁来说，当同一线程去再次获取锁的时候,不用进行CAS操作,提高了性能.(轻量级锁在同一线程情况下每次去获取锁,在无锁的状态下,每次都要进行一次CAS操作)
- 偏向锁只有遇到其他线程尝试竞争偏向锁时,持有偏向锁的线程才会释放锁,线程不会主动去释放偏向锁
- 偏向锁的撤销是很复杂，成为理解代码的障碍，也阻碍了对同步系统重构，而且现如今基本都是多核系统，偏向锁的劣势越来越明显，所以在Java 15废弃了偏向锁
