synchronized

synchronized 三种用法

1. synchronize static method ，synchronized 修饰静态方法
2. synchronize instance method ， synchronized 修饰实例方法
3. synchronize{ //todo } ， synchronized 修饰代码块

相对三种方式的实现方式分析：
java 代码：

public class SyncKeyWord {
    public SyncKeyWord() {
    }
    public static synchronized void staticMethod() {
    }
    public synchronized void instanceMethod() {
    }
    public void codeBlock() {
        String var1 = "obj";
        synchronized("obj") {
            ;
        }
    }
}

字节码文件:

public class keyword.SyncKeyWord
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #4.#21         // java/lang/Object."<init>":()V
   #2 = String             #22            // obj
   #3 = Class              #23            // keyword/SyncKeyWord
   #4 = Class              #24            // java/lang/Object
   #5 = Utf8               <init>
   #6 = Utf8               ()V
   #7 = Utf8               Code
   #8 = Utf8               LineNumberTable
   #9 = Utf8               LocalVariableTable
  #10 = Utf8               this
  #11 = Utf8               Lkeyword/SyncKeyWord;
  #12 = Utf8               staticMethod
  #13 = Utf8               instanceMethod
  #14 = Utf8               codeBlock
  #15 = Utf8               StackMapTable
  #16 = Class              #23            // keyword/SyncKeyWord
  #17 = Class              #24            // java/lang/Object
  #18 = Class              #25            // java/lang/Throwable
  #19 = Utf8               SourceFile
  #20 = Utf8               SyncKeyWord.java
  #21 = NameAndType        #5:#6          // "<init>":()V
  #22 = Utf8               obj
  #23 = Utf8               keyword/SyncKeyWord
  #24 = Utf8               java/lang/Object
  #25 = Utf8               java/lang/Throwable
{
  public keyword.SyncKeyWord();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 9: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lkeyword/SyncKeyWord;
  public static synchronized void staticMethod();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_STATIC, ACC_SYNCHRONIZED  ## 此处标记 静态/同步
    Code:
      stack=0, locals=0, args_size=0
         0: return
      LineNumberTable:
        line 13: 0
  public synchronized void instanceMethod();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_SYNCHRONIZED  ## 此处标记 同步
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 17: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       1     0  this   Lkeyword/SyncKeyWord;
  public void codeBlock();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=1
         0: ldc           #2                  // String obj
         2: dup
         3: astore_1
         4: monitorenter              ## 此处开启 monitor 
         5: aload_1
         6: monitorexit                  ## 退出 monitor  释放锁
         7: goto          15
        10: astore_2
        11: aload_1
        12: monitorexit                  ## 代码异常时 需要退出 monitor 释放锁
        13: aload_2
        14: athrow
        15: return
      Exception table:
         from    to  target type
             5     7    10   any
            10    13    10   any
      LineNumberTable:
        line 20: 0
        line 22: 5
        line 23: 15
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      16     0  this   Lkeyword/SyncKeyWord;
      StackMapTable: number_of_entries = 2
        frame_type = 255 /* full_frame */
          offset_delta = 10
          locals = [ class keyword/SyncKeyWord, class java/lang/Object ]
          stack = [ class java/lang/Throwable ]
        frame_type = 250 /* chop */
          offset_delta = 4
}

对于代码块， Jvm 开启了一个monitor，这个monitor 控制了对象 obj 的对象锁，其他线程要进入，只能等 monitor释放obj的对象锁
对于 实例方法，对方法进行了 同步标记 ，然后通过获取实例对象的对象锁进行控制
对于静态方法，对方法做了 静态同步标记，通过 锁定 Class对象 的对象锁 进行控制
对应synchronized ，其所有的控制都是基于Java 对象 头的 markwork锁标识来操作的。

对象头的组成部分： Markword 类指针 数组长度（数组对象才有）

• markword
  • 32位JVM中markword | 锁状态 | 25bit | | 4bit | 1bit | 2bit | | —- | —- | —- | —- | —- | —- | | | 23bit | 2bit | | 是否偏向锁 | | | 无锁 | 对象Hashcode | | 分代年龄 | 0 | | | 偏向锁 | 线程ID | Epoch | | | | | 轻量级锁 | | | | | | | 重量级锁 | | | | | | | GC标记 | | | | | |

monitor 是由 ObjectMonitor 实现的，

 ObjectMonitor() {
    _header       = NULL;
    _count        = 0;        ## 等待的线程数
    _waiters      = 0,
    _recursions   = 0;
    _object       = NULL;
    _owner        = NULL;     ## 持有锁的线程        
    _WaitSet      = NULL;    ##处于wait的线程，加入到这个set中
    _WaitSetLock  = 0 ;
    _Responsible  = NULL ;
    _succ         = NULL ;
    _cxq          = NULL ;
    FreeNext      = NULL ;
    _EntryList    = NULL ;   ## 处于等待锁block的线程，加入到这里
    _SpinFreq     = 0 ;
    _SpinClock    = 0 ;
    OwnerIsThread = 0 ;
    _previous_owner_tid = 0;
  }

ObjectMonitor 源码

/*
 * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */
#ifndef SHARE_VM_RUNTIME_OBJECTMONITOR_HPP
#define SHARE_VM_RUNTIME_OBJECTMONITOR_HPP
#include "runtime/os.hpp"
#include "runtime/park.hpp"
#include "runtime/perfData.hpp"
// ObjectWaiter serves as a "proxy" or surrogate thread.
// TODO-FIXME: Eliminate ObjectWaiter and use the thread-specific
// ParkEvent instead.  Beware, however, that the JVMTI code
// knows about ObjectWaiters, so we'll have to reconcile that code.
// See next_waiter(), first_waiter(), etc.
class ObjectWaiter : public StackObj {
 public:
  enum TStates { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER, TS_CXQ } ;
  enum Sorted  { PREPEND, APPEND, SORTED } ;
  ObjectWaiter * volatile _next;
  ObjectWaiter * volatile _prev;
  Thread*       _thread;
  jlong         _notifier_tid;
  ParkEvent *   _event;
  volatile int  _notified ;
  volatile TStates TState ;
  Sorted        _Sorted ;           // List placement disposition
  bool          _active ;           // Contention monitoring is enabled
 public:
  ObjectWaiter(Thread* thread);
  void wait_reenter_begin(ObjectMonitor *mon);
  void wait_reenter_end(ObjectMonitor *mon);
};
// WARNING:
//   This is a very sensitive and fragile class. DO NOT make any
// change unless you are fully aware of the underlying semantics.
//   This class can not inherit from any other class, because I have
// to let the displaced header be the very first word. Otherwise I
// have to let markOop include this file, which would export the
// monitor data structure to everywhere.
//
// The ObjectMonitor class is used to implement JavaMonitors which have
// transformed from the lightweight structure of the thread stack to a
// heavy weight lock due to contention
// It is also used as RawMonitor by the JVMTI
class ObjectMonitor {
 public:
  enum {
    OM_OK,                    // no error
    OM_SYSTEM_ERROR,          // operating system error
    OM_ILLEGAL_MONITOR_STATE, // IllegalMonitorStateException
    OM_INTERRUPTED,           // Thread.interrupt()
    OM_TIMED_OUT              // Object.wait() timed out
  };
 public:
  // TODO-FIXME: the "offset" routines should return a type of off_t instead of int ...
  // ByteSize would also be an appropriate type.
  static int header_offset_in_bytes()      { return offset_of(ObjectMonitor, _header);     }
  static int object_offset_in_bytes()      { return offset_of(ObjectMonitor, _object);     }
  static int owner_offset_in_bytes()       { return offset_of(ObjectMonitor, _owner);      }
  static int count_offset_in_bytes()       { return offset_of(ObjectMonitor, _count);      }
  static int recursions_offset_in_bytes()  { return offset_of(ObjectMonitor, _recursions); }
  static int cxq_offset_in_bytes()         { return offset_of(ObjectMonitor, _cxq) ;       }
  static int succ_offset_in_bytes()        { return offset_of(ObjectMonitor, _succ) ;      }
  static int EntryList_offset_in_bytes()   { return offset_of(ObjectMonitor, _EntryList);  }
  static int FreeNext_offset_in_bytes()    { return offset_of(ObjectMonitor, FreeNext);    }
  static int WaitSet_offset_in_bytes()     { return offset_of(ObjectMonitor, _WaitSet) ;   }
  static int Responsible_offset_in_bytes() { return offset_of(ObjectMonitor, _Responsible);}
  static int Spinner_offset_in_bytes()     { return offset_of(ObjectMonitor, _Spinner);    }
 public:
  // Eventaully we'll make provisions for multiple callbacks, but
  // now one will suffice.
  static int (*SpinCallbackFunction)(intptr_t, int) ;
  static intptr_t SpinCallbackArgument ;
 public:
  markOop   header() const;
  void      set_header(markOop hdr);
  intptr_t is_busy() const {
    // TODO-FIXME: merge _count and _waiters.
    // TODO-FIXME: assert _owner == null implies _recursions = 0
    // TODO-FIXME: assert _WaitSet != null implies _count > 0
    return _count|_waiters|intptr_t(_owner)|intptr_t(_cxq)|intptr_t(_EntryList ) ;
  }
  intptr_t  is_entered(Thread* current) const;
  void*     owner() const;
  void      set_owner(void* owner);
  intptr_t  waiters() const;
  intptr_t  count() const;
  void      set_count(intptr_t count);
  intptr_t  contentions() const ;
  intptr_t  recursions() const                                         { return _recursions; }
  // JVM/DI GetMonitorInfo() needs this
  ObjectWaiter* first_waiter()                                         { return _WaitSet; }
  ObjectWaiter* next_waiter(ObjectWaiter* o)                           { return o->_next; }
  Thread* thread_of_waiter(ObjectWaiter* o)                            { return o->_thread; }
  // initialize the monitor, exception the semaphore, all other fields
  // are simple integers or pointers
  ObjectMonitor() {
    _header       = NULL;
    _count        = 0;
    _waiters      = 0,
    _recursions   = 0;
    _object       = NULL;
    _owner        = NULL;
    _WaitSet      = NULL;
    _WaitSetLock  = 0 ;
    _Responsible  = NULL ;
    _succ         = NULL ;
    _cxq          = NULL ;
    FreeNext      = NULL ;
    _EntryList    = NULL ;
    _SpinFreq     = 0 ;
    _SpinClock    = 0 ;
    OwnerIsThread = 0 ;
    _previous_owner_tid = 0;
  }
  ~ObjectMonitor() {
   // TODO: Add asserts ...
   // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
   // _count == 0 _EntryList  == NULL etc
  }
private:
  void Recycle () {
    // TODO: add stronger asserts ...
    // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
    // _count == 0 EntryList  == NULL
    // _recursions == 0 _WaitSet == NULL
    // TODO: assert (is_busy()|_recursions) == 0
    _succ          = NULL ;
    _EntryList     = NULL ;
    _cxq           = NULL ;
    _WaitSet       = NULL ;
    _recursions    = 0 ;
    _SpinFreq      = 0 ;
    _SpinClock     = 0 ;
    OwnerIsThread  = 0 ;
  }
public:
  void*     object() const;
  void*     object_addr();
  void      set_object(void* obj);
  bool      check(TRAPS);       // true if the thread owns the monitor.
  void      check_slow(TRAPS);
  void      clear();
  static void sanity_checks();  // public for -XX:+ExecuteInternalVMTests
                                // in PRODUCT for -XX:SyncKnobs=Verbose=1
#ifndef PRODUCT
  void      verify();
  void      print();
#endif
  bool      try_enter (TRAPS) ;
  void      enter(TRAPS);
  void      exit(bool not_suspended, TRAPS);
  void      wait(jlong millis, bool interruptable, TRAPS);
  void      notify(TRAPS);
  void      notifyAll(TRAPS);
// Use the following at your own risk
  intptr_t  complete_exit(TRAPS);
  void      reenter(intptr_t recursions, TRAPS);
 private:
  void      AddWaiter (ObjectWaiter * waiter) ;
  static    void DeferredInitialize();
  ObjectWaiter * DequeueWaiter () ;
  void      DequeueSpecificWaiter (ObjectWaiter * waiter) ;
  void      EnterI (TRAPS) ;
  void      ReenterI (Thread * Self, ObjectWaiter * SelfNode) ;
  void      UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode) ;
  int       TryLock (Thread * Self) ;
  int       NotRunnable (Thread * Self, Thread * Owner) ;
  int       TrySpin_Fixed (Thread * Self) ;
  int       TrySpin_VaryFrequency (Thread * Self) ;
  int       TrySpin_VaryDuration  (Thread * Self) ;
  void      ctAsserts () ;
  void      ExitEpilog (Thread * Self, ObjectWaiter * Wakee) ;
  bool      ExitSuspendEquivalent (JavaThread * Self) ;
 private:
  friend class ObjectSynchronizer;
  friend class ObjectWaiter;
  friend class VMStructs;
  // WARNING: this must be the very first word of ObjectMonitor
  // This means this class can't use any virtual member functions.
  volatile markOop   _header;       // displaced object header word - mark
  void*     volatile _object;       // backward object pointer - strong root
  double SharingPad [1] ;           // temp to reduce false sharing
  // All the following fields must be machine word aligned
  // The VM assumes write ordering wrt these fields, which can be
  // read from other threads.
 protected:                         // protected for jvmtiRawMonitor
  void *  volatile _owner;          // pointer to owning thread OR BasicLock
  volatile jlong _previous_owner_tid; // thread id of the previous owner of the monitor
  volatile intptr_t  _recursions;   // recursion count, 0 for first entry
 private:
  int OwnerIsThread ;               // _owner is (Thread *) vs SP/BasicLock
  ObjectWaiter * volatile _cxq ;    // LL of recently-arrived threads blocked on entry.
                                    // The list is actually composed of WaitNodes, acting
                                    // as proxies for Threads.
 protected:
  ObjectWaiter * volatile _EntryList ;     // Threads blocked on entry or reentry.
 private:
  Thread * volatile _succ ;          // Heir presumptive thread - used for futile wakeup throttling
  Thread * volatile _Responsible ;
  int _PromptDrain ;                // rqst to drain cxq into EntryList ASAP
  volatile int _Spinner ;           // for exit->spinner handoff optimization
  volatile int _SpinFreq ;          // Spin 1-out-of-N attempts: success rate
  volatile int _SpinClock ;
  volatile int _SpinDuration ;
  volatile intptr_t _SpinState ;    // MCS/CLH list of spinners
  // TODO-FIXME: _count, _waiters and _recursions should be of
  // type int, or int32_t but not intptr_t.  There's no reason
  // to use 64-bit fields for these variables on a 64-bit JVM.
  volatile intptr_t  _count;        // reference count to prevent reclaimation/deflation
                                    // at stop-the-world time.  See deflate_idle_monitors().
                                    // _count is approximately |_WaitSet| + |_EntryList|
 protected:
  volatile intptr_t  _waiters;      // number of waiting threads
 private:
 protected:
  ObjectWaiter * volatile _WaitSet; // LL of threads wait()ing on the monitor
 private:
  volatile int _WaitSetLock;        // protects Wait Queue - simple spinlock
 public:
  int _QMix ;                       // Mixed prepend queue discipline
  ObjectMonitor * FreeNext ;        // Free list linkage
  intptr_t StatA, StatsB ;
 public:
  static void Initialize () ;
  static PerfCounter * _sync_ContendedLockAttempts ;
  static PerfCounter * _sync_FutileWakeups ;
  static PerfCounter * _sync_Parks ;
  static PerfCounter * _sync_EmptyNotifications ;
  static PerfCounter * _sync_Notifications ;
  static PerfCounter * _sync_SlowEnter ;
  static PerfCounter * _sync_SlowExit ;
  static PerfCounter * _sync_SlowNotify ;
  static PerfCounter * _sync_SlowNotifyAll ;
  static PerfCounter * _sync_FailedSpins ;
  static PerfCounter * _sync_SuccessfulSpins ;
  static PerfCounter * _sync_PrivateA ;
  static PerfCounter * _sync_PrivateB ;
  static PerfCounter * _sync_MonInCirculation ;
  static PerfCounter * _sync_MonScavenged ;
  static PerfCounter * _sync_Inflations ;
  static PerfCounter * _sync_Deflations ;
  static PerfLongVariable * _sync_MonExtant ;
 public:
  static int Knob_Verbose;
  static int Knob_SpinLimit;
  void* operator new (size_t size) throw() {
    return AllocateHeap(size, mtInternal);
  }
  void* operator new[] (size_t size) throw() {
    return operator new (size);
  }
  void operator delete(void* p) {
    FreeHeap(p, mtInternal);
  }
  void operator delete[] (void *p) {
    operator delete(p);
  }
};
#undef TEVENT
#define TEVENT(nom) {if (SyncVerbose) FEVENT(nom); }
#define FEVENT(nom) { static volatile int ctr = 0 ; int v = ++ctr ; if ((v & (v-1)) == 0) { ::printf (#nom " : %d \n", v); ::fflush(stdout); }}
#undef  TEVENT
#define TEVENT(nom) {;}
#endif // SHARE_VM_RUNTIME_OBJECTMONITOR_HPP