“Object 类(及其子类)的每个实例都拥有一个锁，该锁在进入 同步 方法时获得，并在退出时自动释放”

这是否意味着我们创建的任何对象实例在内部默认都有一个“锁”(作为字段实现)？

我对这个“锁”概念感到困惑，我想知道它在内部实际上做了什么。

谁能指引我去一些可以找到更多信息的地方？

最佳答案

与往常一样，JLS provides the answer (17.1) :

The most basic of these methods is synchronization, which is implemented using monitors. Each object in Java is associated with a monitor, which a thread can lock or unlock. Only one thread at a time may hold a lock on a monitor. Any other threads attempting to lock that monitor are blocked until they can obtain a lock on that monitor. A thread t may lock a particular monitor multiple times; each unlock reverses the effect of one lock operation.

所以，不，lock 不像 Object 中的字段(您只需查看 Object's source code 即可看到)。相反，每个 Object 都与一个“监视器”相关联，而正是这个监视器被锁定或解锁。

我只是想指出一个进一步的引用资料，其中详细说明了“Java 是如何做到的”，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，我鼓励所有对下面内容的支持来回答他的问题。您可以在此处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 // -----------------------------------------------------------------------------

关于Java 锁概念在内部是如何工作的？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/17368595/