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wait 源码实现如下
//TRAPS 表示是否有异常
void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
if (UseBiasedLocking) {
// 如果是使用了偏向锁，要撤销偏向锁
BiasedLocking::revoke_and_rebias(obj, false, THREAD);
assert(!obj->mark()->has_bias_pattern(), “biases should be revoked by now”);
}
…
在 biasedLocking.cpp 中可以看到方法的实现。整体结构划分如下
BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
//1：必须在安全点
assert(!SafepointSynchronize::is_at_safepoint(), “must not be called while at safepoint”);
//2：读取对象头
markOop mark = obj->mark();
if (mark->is_biased_anonymously() && !attempt_rebias) {
//3: 没有线程获取了偏向锁
} else if (mark->has_bias_pattern()) {
//4: 已经偏向了
}
//5: 没有执行偏向，通过启发式的方式决定到底是执行撤销还是执行 rebias
HeuristicsResult heuristics = update_heuristics(obj(), attempt_rebias);
if (heuristics == HR_NOT_BIASED) {
//5.1: 偏向状态改成了不需要偏向
} else if (heuristics == HR_SINGLE_REVOKE) {
//5.2: 启发式决定执行单次的撤销
}
//6：等到虚拟机运行到 safepoint, 实际就是执行 VM_BulkRevokeBias 的 doit 的 bulk_revoke_or_rebias_at_safepoint 方法
VM_BulkRevokeBias bulk_revoke(&obj, (JavaThread*) THREAD,
(heuristics == HR_BULK_REBIAS),
attempt_rebias);
VMThread::execute(&bulk_revoke);
return bulk_revoke.status_code();
}
没有获取偏向锁
这里表示还没有被偏向，并且不是执行 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;
//prootype 本身构建的是 markOop(no_hash_in_place | no_lock_in_place);
markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
// 执行 CAS, 如果当前对象的 mark 没有变更，就换成 unbiased_prototype
markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
if (res_mark == biased_value) {
// 如果之前的和现在的一样，说明撤销成功，BIAS_REVOKED 本身是一个枚举
return BIAS_REVOKED;
}
已经被其它线程获取了偏向
// 已经被线程偏向了，获取 Klass 对象，即类本身的头，obj 则是它的实例
Klass* k = Klass::cast(obj->klass());
markOop prototype_header = k->prototype_header();
if (!prototype_header->has_bias_pattern()) {
// 对象当前的偏向状态已经过期, 并且是不可偏向的，直接设置成已经撤销偏向即可
// 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()) {
// 实例的 epoch 和类本身的 epoch 值不一样，说明它已经过期，也就是说这个对象当前处于未偏向但是可偏向的状态 (rebiasable)
// 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) {
// 执行 rebias wait 希望直接撤销
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) {
// 当前线程抢到了这个对象的偏向
return BIAS_REVOKED_AND_REBIASED;
}
} else {
markOop biased_value = mark;
markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
//CAS 撤销偏向锁
markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
if (res_mark == biased_value) {
// 撤销了偏向
return BIAS_REVOKED;
}
}
}
启发式策略
// 启发式的方式决定要做那种操作 static HeuristicsResult update_heuristics(oop o, bool allow_rebias) {
markOop mark = o->mark();
if (!mark->has_bias_pattern()) {
// 不可偏向直接返回 return HR_NOT_BIASED;
}
// 控制撤销的次数
// Heuristics to attempt to throttle the number of revocations.
// Stages: // 1. Revoke the biases of all objects in the heap of this type,
// but allow rebiasing of those objects if unlocked.
// 2. Revoke the biases of all objects in the heap of this type
// and don’t allow rebiasing of these objects. Disable
// allocation of objects of that type with the bias bit set. Klass* k = o->blueprint();
jlong cur_time = os::javaTimeMillis();
// 获取上次执行 bulk revication 的时间 jlong last_bulk_revocation_time = k->last_biased_lock_bulk_revocation_time();
// 获取执行 bulk revocation 的次数 int revocation_count = k->biased_lock_revocation_count();
// 定义在 globs.hpp,BiasedLockingBulkRebiasThreshold 取值为 20；BiasedLockingBulkRevokeThreshold 取值为 40,BiasedLockingDecayTime 为 25000 毫秒
if ((revocation_count >= BiasedLockingBulkRebiasThreshold) &&
(revocation_count < BiasedLockingBulkRevokeThreshold) &&
(last_bulk_revocation_time != 0) &&
(cur_time – last_bulk_revocation_time >= BiasedLockingDecayTime)) {
// This is the first revocation we’ve seen in a while of an
// object of this type since the last time we performed a bulk
// rebiasing operation. The application is allocating objects in
// bulk which are biased toward a thread and then handing them
// off to another thread. We can cope with this allocation
// pattern via the bulk rebiasing mechanism so we reset the
// klass’s revocation count rather than allow it to increase
// monotonically. If we see the need to perform another bulk
// rebias operation later, we will, and if subsequently we see
// many more revocation operations in a short period of time we
// will completely disable biasing for this type.
// 在执行了一定时间之内，执行的撤销次数没有超过阈值，那么认为可以优先执行 bulk rebias, 因此将计数回归原始值
k->set_biased_lock_revocation_count(0);
revocation_count = 0;
}
// Make revocation count saturate just beyond BiasedLockingBulkRevokeThreshold
if (revocation_count <= BiasedLockingBulkRevokeThreshold) {
// 计算执行撤销的次数
revocation_count = k->atomic_incr_biased_lock_revocation_count();
}
if (revocation_count == BiasedLockingBulkRevokeThreshold) {
// 达到执行 bulk revoke 的阈值，执行 bulk revoke return HR_BULK_REVOKE;
}
if (revocation_count == BiasedLockingBulkRebiasThreshold) {
// 达到 bulk rebias 的阈值，执行 bulk rebias
return HR_BULK_REBIAS;
}
// 默认执行单次的撤销
return HR_SINGLE_REVOKE;
}
bulk_revoke_or_rebias_at_safepoint
bulk revoke 的关键在于它会遍历所有线程栈的每一帧
static BiasedLocking::Condition bulk_revoke_or_rebias_at_safepoint(oop o,
bool bulk_rebias,
bool attempt_rebias_of_object,
JavaThread* requesting_thread) {
…
if (bulk_rebias) {
…
// Now walk all threads’ stacks and adjust epochs of any biased
// and locked objects of this data type we encounter
// 遍历所有的线程
for (JavaThread* thr = Threads::first(); thr != NULL; thr = thr->next()) {
// 遍历线程栈的每一帧，获取所有的监视器
GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(thr);
for (int i = 0; i < cached_monitor_info->length(); i++) {
MonitorInfo* mon_info = cached_monitor_info->at(i);
oop owner = mon_info->owner();
markOop mark = owner->mark();
if ((owner->klass() == k_o) && mark->has_bias_pattern()) {
// We might have encountered this object already in the case of recursive locking
assert(mark->bias_epoch() == prev_epoch || mark->bias_epoch() == cur_epoch, “error in bias epoch adjustment”);
// 更新所有栈中的有偏向锁的 epoch
owner->set_mark(mark->set_bias_epoch(cur_epoch));
}
}
}
…
// At this point we’re done. All we have to do is potentially// adjust the header of the given object to revoke its bias.
revoke_bias(o, attempt_rebias_of_object && klass->prototype_header()->has_bias_pattern(), true, requesting_thread);
}
…
if (attempt_rebias_of_object &&
o->mark()->has_bias_pattern() &&
klass->prototype_header()->has_bias_pattern()) {
//bias_epoch 本身则是表示获取了偏向锁
markOop new_mark = markOopDesc::encode(requesting_thread, o->mark()->age(),
klass->prototype_header()->bias_epoch());
o->set_mark(new_mark);
// 执行 rebiase
status_code = BiasedLocking::BIAS_REVOKED_AND_REBIASED;
…
}
}
revoke_bias 的执行如下
static BiasedLocking::Condition revoke_bias(oop obj, bool allow_rebias, bool is_bulk, JavaThread* requesting_thread) {
markOop mark = obj->mark();
…
// 偏向锁的头
markOop biased_prototype = markOopDesc::biased_locking_prototype()->set_age(age);
// 非偏向锁的头
markOop unbiased_prototype = markOopDesc::prototype()->set_age(age);
…
// 获取偏向的线程
JavaThread* biased_thread = mark->biased_locker();
if (biased_thread == NULL) {
// Object is anonymously biased. We can get here if, for
// example, we revoke the bias due to an identity hash code
// being computed for an object.
if (!allow_rebias) {
// 没有线程获取，又需要执行 rebias，改掉对象头即可
obj->set_mark(unbiased_prototype);
}
…
// 撤销完毕
return BiasedLocking::BIAS_REVOKED;
}
…
// 线程活着
// 遍历栈帧，获取所有这个线程的监视器, 按照最年轻到最老的顺序
GrowableArray<MonitorInfo*>* cached_monitor_info = get_or_compute_monitor_info(biased_thread);
BasicLock* highest_lock = NULL;
for (int i = 0; i < cached_monitor_info->length(); i++) {
MonitorInfo* mon_info = cached_monitor_info->at(i);
if (mon_info->owner() == obj) {
…
// Assume recursive case and fix up highest lock later
// 当前栈帧存在了这个对象的锁
markOop mark = markOopDesc::encode((BasicLock*) NULL);
highest_lock = mon_info->lock();
// 更新栈中的 mark 为 NULL
highest_lock->set_displaced_header(mark);
}
…
}
}
if (highest_lock != NULL) {
// Fix up highest lock to contain displaced header and point
// object at it
// 将最久的那个 lock 更新为没有偏向 , 栈中 设置了锁记录
highest_lock->set_displaced_header(unbiased_prototype);
// Reset object header to point to displaced mark
// 将对象头指向栈中的位置，这样表示就没有偏向了
obj->set_mark(markOopDesc::encode(highest_lock));
assert(!obj->mark()->has_bias_pattern(), “illegal mark state: stack lock used bias bit”);
…
} else {
…
if (allow_rebias) {
obj->set_mark(biased_prototype);
} else {
// Store the unlocked value into the object’s header.
obj->set_mark(unbiased_prototype);
}
}
// 撤销完毕 return BiasedLocking::BIAS_REVOKED;
}
