Java中synchronized与ReentrantLock性能对比

前两天逛博客的时候看到有个人写了一篇博客说ReentrantLock比synchronized慢，这就很违反我的认知了，详细看了他的博客和测试代码，发现了他测试的不严谨，并在评论中友好地指出了他的问题，结果他直接把博客给删了删了了……

很多老一辈的程序猿对有synchronized有个 性能差 的刻板印象，然后极力推崇使用java.util.concurrent包中的lock类，如果你追问他们synchronized和lock实现性能差多少，估计没几个人能答出来。说到这你是不是也很想知道我的测试结果？ synchronized与ReentrantLock所实现的功能差不多，用途也大幅度重合，索性我们就来测测这二者的性能差异。

实测结果

测试平台：jdk11, MacBook Pro (13-inch, 2017) , jmh测试

测试代码如下：

public class LockTest {

private static Object lock = new Object();
private static ReentrantLock reentrantLock = new ReentrantLock();
private static long cnt = 0;
@Benchmark
@Measurement(iterations = 2)
@Threads(10)
@Fork(0)
@Warmup(iterations = 5, time = 10)
public void testWithoutLock(){
    doSomething();
}
@Benchmark
@Measurement(iterations = 2)
@Threads(10)
@Fork(0)
@Warmup(iterations = 5, time = 10)
public void testReentrantLock(){
    reentrantLock.lock();
    doSomething();
    reentrantLock.unlock();
}
@Benchmark
@Measurement(iterations = 2)
@Threads(10)
@Fork(0)
@Warmup(iterations = 5, time = 10)
public void testSynchronized(){
    synchronized (lock) {
        doSomething();
    }
}
private void doSomething() {
    cnt += 1;
    if (cnt >= (Long.MAX_VALUE >> 1)) {
        cnt = 0;
    }
}
public static void main(String[] args) {
    Options options = new OptionsBuilder().include(LockTest.class.getSimpleName()).build();
    try {
        new Runner(options).run();
    } catch (Exception e) {
    } finally {
    }
}

}

Benchmark Mode Cnt Score Error Units LockTest.testReentrantLock thrpt 2 32283819.289 ops/s LockTest.testSynchronized thrpt 2 25325244.320 ops/s LockTest.testWithoutLock thrpt 2 641215542.492 ops/s

没错synchronized性能确实更差，但就只差20%左右，第一次测试的时候我也挺诧异的，知道synchronized会差，但那种预期中几个数量级的差异却没有出现。于是我又把@Threads线程数调大了，增加了多线程之间竞争的可能性，得到了如下的结果。

Benchmark Mode Cnt Score Error Units LockTest.testReentrantLock thrpt 2 29464798.051 ops/s LockTest.testSynchronized thrpt 2 22346035.066 ops/s LockTest.testWithoutLock thrpt 2 383047064.795 ops/s

性能差异稍有拉开，但还是在同一量级上。

结论

无可置疑，synchronized的性能确实要比synchronized差个20%-30%，那是不是代码中所有用到synchronized的地方都应该换成lock？非也，仔细想想看，ReentrantLock几乎和可以替代任何使用synchronized的场景，而且性能更好，那为什么jdk一直要留着这个关键词呢？而且完全没有任何想要废弃它的想法。

黑格尔说过存在即合理， synchronized因多线程应运而生，它的存在也大幅度简化了Java多线程的开发。没错，它的优势就是使用简单，你不需要显示去加减锁，相比之下ReentrantLock的使用就繁琐的多了，你加完锁之后还得考虑到各种情况下的锁释放，稍不留神就一个bug埋下了。

但ReentrantLock的繁琐之下，它也提供了更复杂的api，足以应对更多更复杂的需求，详细可以参考我之前的博客ReentrantLock源码解析。

如今synchronized与ReentrantLock二者的性能差异不再是选谁的主要因素，你在做选择的时候更应该考虑的是其易用性、功能性和代码的可维护性…… 二者30%的性能差异决定不了什么，如果你真想优化代码的性能，你应该选择的是其他的切入点，而不是斤斤计较这个，切记不要拣了芝麻丢了西瓜。

文章本该到这里就结束了，但我仍然好奇为什么synchronized给老一辈java程序猿留下了性能差的印象，无奈jdk1.5及之前的资料已经比较久远不太好找，但是jdk1.6对synchronized的性能提升做了啥还是很好找的。

jdk对synchronized优化了啥？

如果你对代码段加了synchronized的，jvm编译后就会在其前后分别插入monitorenter和monitorexit指令，如下：

void onlyMe(Foo f) {
    synchronized(f) {
        doSomething();
    }
}

编译后：

Method void onlyMe(Foo)
0   aload_1             // Push f
1   dup                 // Duplicate it on the stack
2   astore_2            // Store duplicate in local variable 2
3   monitorenter        // Enter the monitor associated with f
4   aload_0             // Holding the monitor, pass this and...
5   invokevirtual #5    // ...call Example.doSomething()V
8   aload_2             // Push local variable 2 (f)
9   monitorexit         // Exit the monitor associated with f
10  goto 18             // Complete the method normally
13  astore_3            // In case of any throw, end up here
14  aload_2             // Push local variable 2 (f)
15  monitorexit         // Be sure to exit the monitor!
16  aload_3             // Push thrown value...
17  athrow              // ...and rethrow value to the invoker
18  return              // Return in the normal case
Exception table:
From    To      Target      Type
4       10      13          any
13      16      13          any

加锁和释放锁的性能消耗其实就体现在了 monitorenter和monitorexit两个指令上了，如果是优化性能，肯定也是在这两个指令上优化了。查阅《Java并发编程的艺术》发现，Java6为了减少锁获取和释放带来的性能消耗，引入了锁分级的策略。将锁状态分别分成 无锁、偏向锁、轻量级锁、重量级锁 四个状态，其性能依次递减。但所幸因为局部性的存在，大多数并发情况下偏向锁或者轻量级锁就能满足我们的需求，而且锁只有在竞争严重的情况下才会升级，所以大多数情况下synchronized性能也不会太差。

最后我在jdk11u的源码里找到了monitorenter和monitorexit的x86版本的实现(汇编指令和具体平台相关)献给大家，欢迎有志之士研读下。

//-----------------------------------------------------------------------------
// Synchronization
//
// Note: monitorenter & exit are symmetric routines; which is reflected
//       in the assembly code structure as well
//
// Stack layout:
//
// [expressions  ] <--- rsp               = expression stack top
// ..
// [expressions  ]
// [monitor entry] <--- monitor block top = expression stack bot
// ..
// [monitor entry]
// [frame data   ] <--- monitor block bot
// ...
// [saved rbp    ] <--- rbp
void TemplateTable::monitorenter() {
  transition(atos, vtos);
  // check for NULL object
  __ null_check(rax);
  const Address monitor_block_top(
        rbp, frame::interpreter_frame_monitor_block_top_offset * wordSize);
  const Address monitor_block_bot(
        rbp, frame::interpreter_frame_initial_sp_offset * wordSize);
  const int entry_size = frame::interpreter_frame_monitor_size() * wordSize;
  Label allocated;
  Register rtop = LP64_ONLY(c_rarg3) NOT_LP64(rcx);
  Register rbot = LP64_ONLY(c_rarg2) NOT_LP64(rbx);
  Register rmon = LP64_ONLY(c_rarg1) NOT_LP64(rdx);
  // initialize entry pointer
  __ xorl(rmon, rmon); // points to free slot or NULL
  // find a free slot in the monitor block (result in rmon)
  {
    Label entry, loop, exit;
    __ movptr(rtop, monitor_block_top); // points to current entry,
                                        // starting with top-most entry
    __ lea(rbot, monitor_block_bot);    // points to word before bottom
                                        // of monitor block
    __ jmpb(entry);
    __ bind(loop);
    // check if current entry is used
    __ cmpptr(Address(rtop, BasicObjectLock::obj_offset_in_bytes()), (int32_t) NULL_WORD);
    // if not used then remember entry in rmon
    __ cmovptr(Assembler::equal, rmon, rtop);   // cmov => cmovptr
    // check if current entry is for same object
    __ cmpptr(rax, Address(rtop, BasicObjectLock::obj_offset_in_bytes()));
    // if same object then stop searching
    __ jccb(Assembler::equal, exit);
    // otherwise advance to next entry
    __ addptr(rtop, entry_size);
    __ bind(entry);
    // check if bottom reached
    __ cmpptr(rtop, rbot);
    // if not at bottom then check this entry
    __ jcc(Assembler::notEqual, loop);
    __ bind(exit);
  }
  __ testptr(rmon, rmon); // check if a slot has been found
  __ jcc(Assembler::notZero, allocated); // if found, continue with that one
  // allocate one if there's no free slot
  {
    Label entry, loop;
    // 1. compute new pointers          // rsp: old expression stack top
    __ movptr(rmon, monitor_block_bot); // rmon: old expression stack bottom
    __ subptr(rsp, entry_size);         // move expression stack top
    __ subptr(rmon, entry_size);        // move expression stack bottom
    __ mov(rtop, rsp);                  // set start value for copy loop
    __ movptr(monitor_block_bot, rmon); // set new monitor block bottom
    __ jmp(entry);
    // 2. move expression stack contents
    __ bind(loop);
    __ movptr(rbot, Address(rtop, entry_size)); // load expression stack
                                                // word from old location
    __ movptr(Address(rtop, 0), rbot);          // and store it at new location
    __ addptr(rtop, wordSize);                  // advance to next word
    __ bind(entry);
    __ cmpptr(rtop, rmon);                      // check if bottom reached
    __ jcc(Assembler::notEqual, loop);          // if not at bottom then
                                                // copy next word
  }
  // call run-time routine
  // rmon: points to monitor entry
  __ bind(allocated);
  // Increment bcp to point to the next bytecode, so exception
  // handling for async. exceptions work correctly.
  // The object has already been poped from the stack, so the
  // expression stack looks correct.
  __ increment(rbcp);
  // store object
  __ movptr(Address(rmon, BasicObjectLock::obj_offset_in_bytes()), rax);
  __ lock_object(rmon);
  // check to make sure this monitor doesn't cause stack overflow after locking
  __ save_bcp();  // in case of exception
  __ generate_stack_overflow_check(0);
  // The bcp has already been incremented. Just need to dispatch to
  // next instruction.
  __ dispatch_next(vtos);
}
void TemplateTable::monitorexit() {
  transition(atos, vtos);
  // check for NULL object
  __ null_check(rax);
  const Address monitor_block_top(
        rbp, frame::interpreter_frame_monitor_block_top_offset * wordSize);
  const Address monitor_block_bot(
        rbp, frame::interpreter_frame_initial_sp_offset * wordSize);
  const int entry_size = frame::interpreter_frame_monitor_size() * wordSize;
  Register rtop = LP64_ONLY(c_rarg1) NOT_LP64(rdx);
  Register rbot = LP64_ONLY(c_rarg2) NOT_LP64(rbx);
  Label found;
  // find matching slot
  {
    Label entry, loop;
    __ movptr(rtop, monitor_block_top); // points to current entry,
                                        // starting with top-most entry
    __ lea(rbot, monitor_block_bot);    // points to word before bottom
                                        // of monitor block
    __ jmpb(entry);
    __ bind(loop);
    // check if current entry is for same object
    __ cmpptr(rax, Address(rtop, BasicObjectLock::obj_offset_in_bytes()));
    // if same object then stop searching
    __ jcc(Assembler::equal, found);
    // otherwise advance to next entry
    __ addptr(rtop, entry_size);
    __ bind(entry);
    // check if bottom reached
    __ cmpptr(rtop, rbot);
    // if not at bottom then check this entry
    __ jcc(Assembler::notEqual, loop);
  }