问题描述
类Object(及其子类)的每个实例都拥有一个锁定,即在进入同步方法时获得
并在退出时自动释放
"Every instance of class Object (and its subclasses) possesses a lock thatis obtained on entry to a synchronized method and automatically released upon exit"
这是否意味着我们内部创建的任何对象实例默认具有锁定(实现为字段)?
Does this mean that any object instance we create internally having a "lock" by default (implemented as a field) ?
我对此锁定概念感到困惑,我想知道内部实际上做了什么。
I have got confused with this "lock" concept , I want to know what is actually does internally.
有人可以指引我去找更多信息吗?
Could anyone direct me to some where find more info?
推荐答案
与往常一样,:
所以,不,锁定
不像对象
中的字段(正如您只需查看)。相反,每个对象
都与监视器相关联,并且此监视器被锁定或解锁。
So, no, lock
is not like a field in the Object
(as you can see by simply looking at Object's source code). Rather, each Object
is associated with a "monitor", and it is this monitor which is locked or unlocked.
我只是想指出一个进一步的参考资料,详细说明Java如何做到,以确保它不被忽视。这位于@selig在下面发现的C ++代码的注释中,我鼓励所有上述内容的upvotes回答他的答案。您可以在那里提供的链接中查看完整的源代码。
I just wanted to point out a further reference which details "how Java does it" to make sure it's not overlooked. This is located in the comments of the C++ code which @selig discovered below, and I encourage all upvotes for the content below to go to his answer. You can view the full source code in the link provided there.
126 // -----------------------------------------------------------------------------
127 // Theory of operations -- Monitors lists, thread residency, etc:
128 //
129 // * A thread acquires ownership of a monitor by successfully
130 // CAS()ing the _owner field from null to non-null.
131 //
132 // * Invariant: A thread appears on at most one monitor list --
133 // cxq, EntryList or WaitSet -- at any one time.
134 //
135 // * Contending threads "push" themselves onto the cxq with CAS
136 // and then spin/park.
137 //
138 // * After a contending thread eventually acquires the lock it must
139 // dequeue itself from either the EntryList or the cxq.
140 //
141 // * The exiting thread identifies and unparks an "heir presumptive"
142 // tentative successor thread on the EntryList. Critically, the
143 // exiting thread doesn't unlink the successor thread from the EntryList.
144 // After having been unparked, the wakee will recontend for ownership of
145 // the monitor. The successor (wakee) will either acquire the lock or
146 // re-park itself.
147 //
148 // Succession is provided for by a policy of competitive handoff.
149 // The exiting thread does _not_ grant or pass ownership to the
150 // successor thread. (This is also referred to as "handoff" succession").
151 // Instead the exiting thread releases ownership and possibly wakes
152 // a successor, so the successor can (re)compete for ownership of the lock.
153 // If the EntryList is empty but the cxq is populated the exiting
154 // thread will drain the cxq into the EntryList. It does so by
155 // by detaching the cxq (installing null with CAS) and folding
156 // the threads from the cxq into the EntryList. The EntryList is
157 // doubly linked, while the cxq is singly linked because of the
158 // CAS-based "push" used to enqueue recently arrived threads (RATs).
159 //
160 // * Concurrency invariants:
161 //
162 // -- only the monitor owner may access or mutate the EntryList.
163 // The mutex property of the monitor itself protects the EntryList
164 // from concurrent interference.
165 // -- Only the monitor owner may detach the cxq.
166 //
167 // * The monitor entry list operations avoid locks, but strictly speaking
168 // they're not lock-free. Enter is lock-free, exit is not.
169 // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
170 //
171 // * The cxq can have multiple concurrent "pushers" but only one concurrent
172 // detaching thread. This mechanism is immune from the ABA corruption.
173 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
174 //
175 // * Taken together, the cxq and the EntryList constitute or form a
176 // single logical queue of threads stalled trying to acquire the lock.
177 // We use two distinct lists to improve the odds of a constant-time
178 // dequeue operation after acquisition (in the ::enter() epilog) and
179 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
180 // A key desideratum is to minimize queue & monitor metadata manipulation
181 // that occurs while holding the monitor lock -- that is, we want to
182 // minimize monitor lock holds times. Note that even a small amount of
183 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
184 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
185 // locks and monitor metadata.
186 //
187 // Cxq points to the the set of Recently Arrived Threads attempting entry.
188 // Because we push threads onto _cxq with CAS, the RATs must take the form of
189 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
190 // the unlocking thread notices that EntryList is null but _cxq is != null.
191 //
192 // The EntryList is ordered by the prevailing queue discipline and
193 // can be organized in any convenient fashion, such as a doubly-linked list or
194 // a circular doubly-linked list. Critically, we want insert and delete operations
195 // to operate in constant-time. If we need a priority queue then something akin
196 // to Solaris' sleepq would work nicely. Viz.,
197 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
198 // Queue discipline is enforced at ::exit() time, when the unlocking thread
199 // drains the cxq into the EntryList, and orders or reorders the threads on the
200 // EntryList accordingly.
201 //
202 // Barring "lock barging", this mechanism provides fair cyclic ordering,
203 // somewhat similar to an elevator-scan.
204 //
205 // * The monitor synchronization subsystem avoids the use of native
206 // synchronization primitives except for the narrow platform-specific
207 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
208 // the semantics of park-unpark. Put another way, this monitor implementation
209 // depends only on atomic operations and park-unpark. The monitor subsystem
210 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
211 // underlying OS manages the READY<->RUN transitions.
212 //
213 // * Waiting threads reside on the WaitSet list -- wait() puts
214 // the caller onto the WaitSet.
215 //
216 // * notify() or notifyAll() simply transfers threads from the WaitSet to
217 // either the EntryList or cxq. Subsequent exit() operations will
218 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
219 // it's likely the notifyee would simply impale itself on the lock held
220 // by the notifier.
221 //
222 // * An interesting alternative is to encode cxq as (List,LockByte) where
223 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
224 // variable, like _recursions, in the scheme. The threads or Events that form
225 // the list would have to be aligned in 256-byte addresses. A thread would
226 // try to acquire the lock or enqueue itself with CAS, but exiting threads
227 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
228 // Note that is is *not* word-tearing, but it does presume that full-word
229 // CAS operations are coherent with intermix with STB operations. That's true
230 // on most common processors.
231 //
232 // * See also http://blogs.sun.com/dave
233
234
235 // -----------------------------------------------------------------------------
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