“类Object（及其子类）的每个实例都拥有一个锁 在进入同步方法时获得并在退出时自动释放“
这是否意味着我们在内部创建的任何对象实例默认具有“锁定”（实现为字段）？
我对这个“锁定”概念感到困惑，我想知道内部实际上做了什么。
有人可以指引我到一些找到更多信息吗？
答案 0 :(得分：21)
与往常一样，JLS provides the answer (17.1)：
这些方法中最基本的是同步，它是使用监视器实现的。 Java中的每个对象都与一个监视器相关联，一个线程可以锁定或解锁。一次只有一个线程可以锁定监视器。尝试锁定该监视器的任何其他线程都将被阻止，直到它们可以获得该监视器上的锁定为止。线程t可以多次锁定特定监视器;每次解锁都会逆转一次锁定操作的效果。
所以，不，lock与Object中的字段不同（正如您只需查看Object's source code即可看到）。相反，每个Object都与“监视器”相关联，并且此监视器被锁定或解锁。
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”，以确保它不会被忽视。这位于@selig在下面发现的C ++代码的注释中，我鼓励所有关于下面内容的upvotes转到他的答案。您可以在那里提供的链接中查看完整的源代码。
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 // -----------------------------------------------------------------------------
答案 1 :(得分：20)
另一个答案描述了语言定义所说的内容，而不是“内部发生”的内容。
Java中的每个对象都有一个双字对象标题。标记词和klass指针。第一个单词（标记词）用于存储锁定信息和缓存哈希码。第二个单词是指向klass对象的指针，该对象存储该对象的静态信息（包括方法代码）。
HotSpot JVM有一些花哨的锁定东西，包括瘦锁和偏向锁定，这基本上意味着如果你永远不会锁定一个对象或者你永远不会有任何争用，那么你永远不会创建一个监视器对象（这是存储额外锁定信息的东西） ）。
监视器对象具有条目集。如果对象已被锁定，则锁定该对象时，您的线程将添加到条目集中。解锁对象时，您将在条目集中唤醒一个线程。
并发是一个非常复杂的领域，显然还有很多细节。
<强>更新
答案 2 :(得分：-2)
Lock是Java Concurrency的内部概念。您可以通过同步或Externatl锁获得它。要了解详情，请参阅：http://docs.oracle.com/javase/tutorial/essential/concurrency/newlocks.html