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深入淺析java 中volatile與lock的原理？針對這個問題，這篇文章詳細介紹了相對應的分析和解答，希望可以幫助更多想解決這個問題的小伙伴找到更簡單易行的方法。
java 中volatile和lock原理分析
volatile和lock是Java中用于線程協同同步的兩種機制。
Volatile
volatile是Java中的一個關鍵字，它的作用有
volatile在Java語言規范中規定的是
The Java programming language allows threads to access shared variables (§17.1). As a rule, to ensure that shared variables are consistently and reliably updated, a thread should ensure that it has exclusive use of such variables by obtaining a lock that, conventionally, enforces mutual exclusion for those shared variables. The Java programming language provides a second mechanism, volatile fields, that is more convenient than locking for some purposes. A field may be declared volatile, in which case the Java Memory Model ensures that all threads see a consistent value for the variable . It is a compile-time error if a final variable is also declared volatile.
Java內存模型中規定了volatile的happen-before效果，對volatile變量的寫操作happen-before于后續的讀。這樣volatile變量能夠確保一個線程的修改對其他線程可見。volatile因為不能保證原子性，所以不能在有約束或后驗條件的場景下使用，如i++,常用的場景是stop變量保證系統停止對其他線程可見，double-check lock單例中防止重排序來保證安全發布等。
以下面這段代碼為例
public class TestVolatile {
private static volatile boolean stop = false;
public static void main(String[] args) {
stop = true;
boolean b = stop;
}
}stop字段聲明為volatile類型后，編譯后的字節碼中其變量的access_flag中ACC_VOLATILE位置為1。
關鍵的字節碼內容如下
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=2, args_size=1
0: iconst_1
1: putstatic #2 // Field stop:Z
4: getstatic #2 // Field stop:Z
7: istore_1
8: return
LineNumberTable:
line 14: 0
line 15: 4
line 16: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 args [Ljava/lang/String;
8 1 1 b Z
static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: iconst_0
1: putstatic #2 // Field stop:Z
4: return
LineNumberTable:
line 11: 0
}通過hsdis查看虛擬機產生的匯編代碼。
測試環境為java version “1.8.0_45”,MACOS10.12.1 i386:x86-64
在執行參數上添加
-XX:+UnlockDiagnosticVMOptions -XX:+LogCompilation -XX:+PrintAssembly -Xcomp -XX:CompileCommand=dontinline,*TestVolatile.main -XX:CompileCommand=compileonly,*TestVolatile.main
查看main方法的匯編指令結果
Decoding compiled method 0x000000010c732c50:
Code:
[Disassembling for mach='i386:x86-64']
[Entry Point]
[Verified Entry Point]
[Constants]
# {method} {0x000000012422a2c8} 'main' '([Ljava/lang/String;)V' in 'com/concurrent/volatiles/TestVolatile'
# parm0: rsi:rsi = '[Ljava/lang/String;'
# [sp+0x40] (sp of caller)
0x000000010c732da0: mov %eax,-0x14000(%rsp)
0x000000010c732da7: push %rbp
0x000000010c732da8: sub $0x30,%rsp
0x000000010c732dac: movabs $0x12422a448,%rdi ; {metadata(method data for {method} {0x000000012422a2c8} 'main' '([Ljava/lang/String;)V' in 'com/concurrent/volatiles/TestVolatile')}
0x000000010c732db6: mov 0xdc(%rdi),%ebx
0x000000010c732dbc: add $0x8,%ebx
0x000000010c732dbf: mov %ebx,0xdc(%rdi)
0x000000010c732dc5: movabs $0x12422a2c8,%rdi ; {metadata({method} {0x000000012422a2c8} 'main' '([Ljava/lang/String;)V' in 'com/concurrent/volatiles/TestVolatile')}
0x000000010c732dcf: and $0x0,%ebx
0x000000010c732dd2: cmp $0x0,%ebx
0x000000010c732dd5: je 0x000000010c732e03 ;*iconst_1
; - com.concurrent.volatiles.TestVolatile::main@0 (line 14)
0x000000010c732ddb: movabs $0x76adce798,%rsi ; {oop(a 'java/lang/Class' = 'com/concurrent/volatiles/TestVolatile')}
0x000000010c732de5: mov $0x1,%edi
0x000000010c732dea: mov %dil,0x68(%rsi)
0x000000010c732dee: lock addl $0x0,(%rsp) ;*putstatic stop
; - com.concurrent.volatiles.TestVolatile::main@1 (line 14)
0x000000010c732df3: movsbl 0x68(%rsi),%esi ;*getstatic stop
; - com.concurrent.volatiles.TestVolatile::main@4 (line 15)
0x000000010c732df7: add $0x30,%rsp
0x000000010c732dfb: pop %rbp
0x000000010c732dfc: test %eax,-0x3adbd02(%rip) # 0x0000000108c57100
; {poll_return}
0x000000010c732e02: retq
0x000000010c732e03: mov %rdi,0x8(%rsp)
0x000000010c732e08: movq $0xffffffffffffffff,(%rsp)
0x000000010c732e10: callq 0x000000010c7267e0 ; OopMap{rsi=Oop off=117}
;*synchronization entry
; - com.concurrent.volatiles.TestVolatile::main@-1 (line 14)
; {runtime_call}
0x000000010c732e15: jmp 0x000000010c732ddb
0x000000010c732e17: nop
0x000000010c732e18: nop
0x000000010c732e19: mov 0x2a8(%r15),%rax
0x000000010c732e20: movabs $0x0,%r10
0x000000010c732e2a: mov %r10,0x2a8(%r15)
0x000000010c732e31: movabs $0x0,%r10
0x000000010c732e3b: mov %r10,0x2b0(%r15)
0x000000010c732e42: add $0x30,%rsp
0x000000010c732e46: pop %rbp
0x000000010c732e47: jmpq 0x000000010c6940e0 ; {runtime_call}
[Exception Handler]可以看到在mov %dil,0x68(%rsi)給stop賦值后增加了lock addl $0x0,(%rsp)
IA32中對lock的說明是
The LOCK # signal is asserted during execution of the instruction following the lock prefix. This signal can be used in a multiprocessor system to ensure exclusive use of shared memory while LOCK # is asserted
lock用于在多處理器中執行指令時對共享內存的獨占使用。它的副作用是能夠將當前處理器對應緩存的內容刷新到內存，并使其他處理器對應的緩存失效。另外還提供了有序的指令無法越過這個內存屏障的作用。
Lock
Java中提供的鎖的關鍵字是synchronized, 可以加在方法塊上，也可以加在方法聲明中。
synchronized關鍵字起到的作用是設置一個獨占訪問臨界區，在進入這個臨界區前要先獲取對應的監視器鎖，任何Java對象都可以成為監視器鎖，聲明在靜態方法上時監視器鎖是當前類的Class對象，實例方法上是當前實例。
synchronized提供了原子性、可見性和防止重排序的保證。
JMM中定義監視器鎖的釋放操作happen-before與后續的同一個監視器鎖獲取操作。再結合程序順序規則就可以形成內存傳遞可見性保證。
下面以一段代碼查看各個層次的實現
public class TestSynchronize {
private int count;
private void inc() {
synchronized (this) {
count++;
}
}
public static void main(String[] args) {
new TestSynchronize().inc();
}
}編譯后inc方法的字節碼為
private void inc();
descriptor: ()V
flags: ACC_PRIVATE
Code:
stack=3, locals=3, args_size=1
0: aload_0
1: dup
2: astore_1
3: monitorenter
4: aload_0
5: dup
6: getfield #2 // Field count:I
9: iconst_1
10: iadd
11: putfield #2 // Field count:I
14: aload_1
15: monitorexit
16: goto 24
19: astore_2
20: aload_1
21: monitorexit
22: aload_2
23: athrow
24: return
Exception table:
from to target type
4 16 19 any
19 22 19 any
LineNumberTable:
line 14: 0
line 15: 4在synchronized代碼塊前后增加的monitorenter和monitorexist兩個JVM字節碼指令,指令的參數是this引用。
hotspot中對于monitor_enter和monitor_exit的處理是
void LIRGenerator::monitor_enter(LIR_Opr object, LIR_Opr lock, LIR_Opr hdr, LIR_Opr scratch, int monitor_no, CodeEmitInfo* info_for_exception, CodeEmitInfo* info) {
if (!GenerateSynchronizationCode) return;
// for slow path, use debug info for state after successful locking
CodeStub* slow_path = new MonitorEnterStub(object, lock, info);
__ load_stack_address_monitor(monitor_no, lock);
// for handling NullPointerException, use debug info representing just the lock stack before this monitorenter
__ lock_object(hdr, object, lock, scratch, slow_path, info_for_exception);
}
void LIRGenerator::monitor_exit(LIR_Opr object, LIR_Opr lock, LIR_Opr new_hdr, LIR_Opr scratch, int monitor_no) {
if (!GenerateSynchronizationCode) return;
// setup registers
LIR_Opr hdr = lock;
lock = new_hdr;
CodeStub* slow_path = new MonitorExitStub(lock, UseFastLocking, monitor_no);
__ load_stack_address_monitor(monitor_no, lock);
__ unlock_object(hdr, object, lock, scratch, slow_path);
}inc方法在本機上輸出的匯編代碼為
Decoding compiled method 0x0000000115be3e50:
Code:
[Entry Point]
[Constants]
# {method} {0x0000000113082328} 'inc' '()V' in 'com/concurrent/lock/TestSynchronize'
# [sp+0x50] (sp of caller)
0x0000000115be3fc0: mov 0x8(%rsi),%r10d
0x0000000115be3fc4: shl $0x3,%r10
0x0000000115be3fc8: cmp %rax,%r10
0x0000000115be3fcb: jne 0x0000000115b1de20 ; {runtime_call}
0x0000000115be3fd1: data32 data32 nopw 0x0(%rax,%rax,1)
0x0000000115be3fdc: data32 data32 xchg %ax,%ax
[Verified Entry Point]
0x0000000115be3fe0: mov %eax,-0x14000(%rsp)
0x0000000115be3fe7: push %rbp
0x0000000115be3fe8: sub $0x40,%rsp
0x0000000115be3fec: movabs $0x113082848,%rax ; {metadata(method data for {method} {0x0000000113082328} 'inc' '()V' in 'com/concurrent/lock/TestSynchronize')}
0x0000000115be3ff6: mov 0xdc(%rax),%edi
0x0000000115be3ffc: add $0x8,%edi
0x0000000115be3fff: mov %edi,0xdc(%rax)
0x0000000115be4005: movabs $0x113082328,%rax ; {metadata({method} {0x0000000113082328} 'inc' '()V' in 'com/concurrent/lock/TestSynchronize')}
0x0000000115be400f: and $0x0,%edi
0x0000000115be4012: cmp $0x0,%edi
0x0000000115be4015: je 0x0000000115be418d ;*aload_0
; - com.concurrent.lock.TestSynchronize::inc@0 (line 14)
0x0000000115be401b: lea 0x20(%rsp),%rdi
0x0000000115be4020: mov %rsi,0x8(%rdi)
0x0000000115be4024: mov (%rsi),%rax
0x0000000115be4027: mov %rax,%rbx
0x0000000115be402a: and $0x7,%rbx
0x0000000115be402e: cmp $0x5,%rbx
0x0000000115be4032: jne 0x0000000115be40b9
0x0000000115be4038: mov 0x8(%rsi),%ebx
0x0000000115be403b: shl $0x3,%rbx
0x0000000115be403f: mov 0xa8(%rbx),%rbx
0x0000000115be4046: or %r15,%rbx
0x0000000115be4049: xor %rax,%rbx
0x0000000115be404c: and $0xffffffffffffff87,%rbx
0x0000000115be4050: je 0x0000000115be40e1
0x0000000115be4056: test $0x7,%rbx
0x0000000115be405d: jne 0x0000000115be40a6
0x0000000115be405f: test $0x300,%rbx
0x0000000115be4066: jne 0x0000000115be4085
0x0000000115be4068: and $0x37f,%rax
0x0000000115be406f: mov %rax,%rbx
0x0000000115be4072: or %r15,%rbx
0x0000000115be4075: lock cmpxchg %rbx,(%rsi)
0x0000000115be407a: jne 0x0000000115be41a4
0x0000000115be4080: jmpq 0x0000000115be40e1
0x0000000115be4085: mov 0x8(%rsi),%ebx
0x0000000115be4088: shl $0x3,%rbx
0x0000000115be408c: mov 0xa8(%rbx),%rbx
0x0000000115be4093: or %r15,%rbx
0x0000000115be4096: lock cmpxchg %rbx,(%rsi)
0x0000000115be409b: jne 0x0000000115be41a4
0x0000000115be40a1: jmpq 0x0000000115be40e1
0x0000000115be40a6: mov 0x8(%rsi),%ebx
0x0000000115be40a9: shl $0x3,%rbx
0x0000000115be40ad: mov 0xa8(%rbx),%rbx
0x0000000115be40b4: lock cmpxchg %rbx,(%rsi)
0x0000000115be40b9: mov (%rsi),%rax
0x0000000115be40bc: or $0x1,%rax
0x0000000115be40c0: mov %rax,(%rdi)
0x0000000115be40c3: lock cmpxchg %rdi,(%rsi)
0x0000000115be40c8: je 0x0000000115be40e1
0x0000000115be40ce: sub %rsp,%rax
0x0000000115be40d1: and $0xfffffffffffff007,%rax
0x0000000115be40d8: mov %rax,(%rdi)
0x0000000115be40db: jne 0x0000000115be41a4 ;*monitorenter
; - com.concurrent.lock.TestSynchronize::inc@3 (line 14)
0x0000000115be40e1: mov 0xc(%rsi),%eax ;*getfield count
; - com.concurrent.lock.TestSynchronize::inc@6 (line 15)
0x0000000115be40e4: inc %eax
0x0000000115be40e6: mov %eax,0xc(%rsi) ;*putfield count
; - com.concurrent.lock.TestSynchronize::inc@11 (line 15)
0x0000000115be40e9: lea 0x20(%rsp),%rax
0x0000000115be40ee: mov 0x8(%rax),%rdi
0x0000000115be40f2: mov (%rdi),%rsi
0x0000000115be40f5: and $0x7,%rsi
0x0000000115be40f9: cmp $0x5,%rsi
0x0000000115be40fd: je 0x0000000115be411a
0x0000000115be4103: mov (%rax),%rsi
0x0000000115be4106: test %rsi,%rsi
0x0000000115be4109: je 0x0000000115be411a
0x0000000115be410f: lock cmpxchg %rsi,(%rdi)
0x0000000115be4114: jne 0x0000000115be41b7 ;*monitorexit
; - com.concurrent.lock.TestSynchronize::inc@15 (line 16)
0x0000000115be411a: movabs $0x113082848,%rax ; {metadata(method data for {method} {0x0000000113082328} 'inc' '()V' in 'com/concurrent/lock/TestSynchronize')}
0x0000000115be4124: incl 0x108(%rax) ;*goto
; - com.concurrent.lock.TestSynchronize::inc@16 (line 16)
0x0000000115be412a: add $0x40,%rsp
0x0000000115be412e: pop %rbp
0x0000000115be412f: test %eax,-0x684e035(%rip) # 0x000000010f396100
; {poll_return}
0x0000000115be4135: retq ;*return
; - com.concurrent.lock.TestSynchronize::inc@24 (line 17)
0x0000000115be4136: mov 0x2a8(%r15),%rax
0x0000000115be413d: xor %r10,%r10
0x0000000115be4140: mov %r10,0x2a8(%r15)
0x0000000115be4147: xor %r10,%r10
0x0000000115be414a: mov %r10,0x2b0(%r15)
0x0000000115be4151: mov %rax,%rsi
0x0000000115be4154: lea 0x20(%rsp),%rax
0x0000000115be4159: mov 0x8(%rax),%rbx
0x0000000115be415d: mov (%rbx),%rdi
0x0000000115be4160: and $0x7,%rdi
0x0000000115be4164: cmp $0x5,%rdi
0x0000000115be4168: je 0x0000000115be4185
0x0000000115be416e: mov (%rax),%rdi
0x0000000115be4171: test %rdi,%rdi
0x0000000115be4174: je 0x0000000115be4185
0x0000000115be417a: lock cmpxchg %rdi,(%rbx)
0x0000000115be417f: jne 0x0000000115be41ca ;*monitorexit
; - com.concurrent.lock.TestSynchronize::inc@21 (line 16)
0x0000000115be4185: mov %rsi,%rax
0x0000000115be4188: jmpq 0x0000000115be4205
0x0000000115be418d: mov %rax,0x8(%rsp)
0x0000000115be4192: movq $0xffffffffffffffff,(%rsp)
0x0000000115be419a: callq 0x0000000115bd5be0 ; OopMap{rsi=Oop off=479}
;*synchronization entry
; - com.concurrent.lock.TestSynchronize::inc@-1 (line 14)
; {runtime_call}
0x0000000115be419f: jmpq 0x0000000115be401b
0x0000000115be41a4: mov %rsi,0x8(%rsp)
0x0000000115be41a9: mov %rdi,(%rsp)
0x0000000115be41ad: callq 0x0000000115bd4060 ; OopMap{rsi=Oop [40]=Oop off=498}
;*monitorenter
; - com.concurrent.lock.TestSynchronize::inc@3 (line 14)
; {runtime_call}
0x0000000115be41b2: jmpq 0x0000000115be40e1
0x0000000115be41b7: lea 0x20(%rsp),%rax
0x0000000115be41bc: mov %rax,(%rsp)
0x0000000115be41c0: callq 0x0000000115bd4420 ; {runtime_call}
0x0000000115be41c5: jmpq 0x0000000115be411a
0x0000000115be41ca: lea 0x20(%rsp),%rax
0x0000000115be41cf: mov %rax,(%rsp)
0x0000000115be41d3: callq 0x0000000115bd4420 ; {runtime_call}
0x0000000115be41d8: jmp 0x0000000115be4185
0x0000000115be41da: nop
0x0000000115be41db: nop
0x0000000115be41dc: mov 0x2a8(%r15),%rax
0x0000000115be41e3: movabs $0x0,%r10
0x0000000115be41ed: mov %r10,0x2a8(%r15)
0x0000000115be41f4: movabs $0x0,%r10
0x0000000115be41fe: mov %r10,0x2b0(%r15)
0x0000000115be4205: add $0x40,%rsp
0x0000000115be4209: pop %rbp
0x0000000115be420a: jmpq 0x0000000115b440e0 ; {runtime_call}
[Exception Handler]其中lock cmpxchg為Compare And Exchange
CMPXCHG compares its destination (first) operand to the value in AL, AX or EAX (depending on the size of the instruction). If they are equal, it copies its source (second) operand into the destination and sets the zero flag. Otherwise, it clears the zero flag and leaves the destination alone.
CMPXCHG is intended to be used for atomic operations in multitasking or multiprocessor environments. To safely update a value in shared memory, for example, you might load the value into EAX, load the updated value into EBX, and then execute the instruction lock cmpxchg [value],ebx. If value has not changed since being loaded, it is updated with your desired new value, and the zero flag is set to let you know it has worked. (The LOCK prefix prevents another processor doing anything in the middle of this operation: it guarantees atomicity.) However, if another processor has modified the value in between your load and your attempted store, the store does not happen, and you are notified of the failure by a cleared zero flag, so you can go round and try again.
關于深入淺析java 中volatile與lock的原理問題的解答就分享到這里了，希望以上內容可以對大家有一定的幫助，如果你還有很多疑惑沒有解開，可以關注億速云行業資訊頻道了解更多相關知識。
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