synchronized的底层JVM实现机制

Java synchronized 包含两方面的含义

互斥

JVM 通过对象锁来实现互斥：

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typedef struct object { uintptr_t lock; Class *class; } Object

typedef struct object {
uintptr_t lock;
Class *class;
} Object

协作

协作是通过 Object wait, notify/notifyAll 方法实现的。

对应到JVM 的底层术语，这一机制叫做 monitor:

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typedef struct monitor { pthread_mutex_t lock; Thread *owner; Object *obj; int count; int in_wait; uintptr_t entering; int wait_count; Thread *wait_set; struct monitor *next; } Monitor;

typedef struct monitor {
pthread_mutex_t lock;
Thread *owner;
Object *obj;
int count;
int in_wait;
uintptr_t entering;
int wait_count;
Thread *wait_set;
struct monitor *next;
} Monitor;

在不同区域的 Thread 对应的状态（Java Thread 的状态图）：



在底层的JVM 或许会有更多的状态，但是，暴露在 Java Thread 的状态是在 java.lang.Thread 中定义的，注意 JDK 6 修订了 interrupt 的语义。

比如，在底层，进行获取 lock 动作到获得 lock 之间有一小段状态叫做 BLOCKED:

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void monitorLock(Monitor *mon, Thread *self) { if(mon->owner == self) mon->count++; else { if(pthread_mutex_trylock(&mon->lock)) { disableSuspend(self); self->blocked_mon = mon; self->blocked_count++; self->state = BLOCKED; pthread_mutex_lock(&mon->lock); self->state = RUNNING; self->blocked_mon = NULL; enableSuspend(self); } mon->owner = self; } }

void monitorLock(Monitor *mon, Thread *self) {
if(mon->owner == self)
mon->count++;
else {
if(pthread_mutex_trylock(&mon->lock)) {
disableSuspend(self);

self->blocked_mon = mon;
self->blocked_count++;
self->state = BLOCKED;

pthread_mutex_lock(&mon->lock);

self->state = RUNNING;
self->blocked_mon = NULL;

enableSuspend(self);
}
mon->owner = self;
}
}