JAVA程序实例：从JDK源码角度看Object-职坐标

JAVA程序实例：从JDK源码角度看Object

小职 2017-11-28 来源：网络阅读 629 评论 0

摘要：讲解JAVA程序实例之从JDK源码角度看Object。Java的Object是所有其他类的父类，从继承的层次来看它就是最顶层根，所以它也是唯一一个没有父类的类。

讲解JAVA程序实例之从JDK源码角度看Object。Java的Object是所有其他类的父类，从继承的层次来看它就是最顶层根，所以它也是唯一一个没有父类的类。它包含了对象常用的一些方法，比如getClass、hashCode、equals、clone、toString、notify、wait等常用方法。所以其他类继承了Object后就可以不用重复实现这些方法。这些方法大多数是native方法，下面具体分析。

主要的代码如下：

public class Object {

private static native void registerNatives();

static {

registerNatives();

}

public final native Class<?> getClass();

public native int hashCode();

public boolean equals(Object obj) {

return (this == obj);

}

protected native Object clone() throws CloneNotSupportedException;

public String toString() {

return getClass().getName() + "@" + Integer.toHexString(hashCode());

}

public final native void notify();

public final native void notifyAll();

public final native void wait(long timeout) throws InterruptedException;

public final void wait(long timeout, int nanos) throws InterruptedException {

if (timeout < 0) {

throw new IllegalArgumentException("timeout value is negative");

}

if (nanos < 0 || nanos > 999999) {

throw new IllegalArgumentException("nanosecond timeout value out of range");

}

if (nanos > 0) {

timeout++;

}

wait(timeout);

}

public final void wait() throws InterruptedException {

wait(0);

}

protected void finalize() throws Throwable {}

}

registerNatives方法

由于registerNatives方法被static块修饰，所以在加载Object类时就会执行该方法，对应的本地方法为Java_java_lang_Object_registerNatives，如下，

JNIEXPORT void JNICALLJava_java_lang_Object_registerNatives(JNIEnv *env, jclass cls){

(*env)->RegisterNatives(env, cls,

methods, sizeof(methods)/sizeof(methods[0]));

}

可以看到它间接调用了JNINativeInterface_结构体的方法，简单可以看成是这样：它干的事大概就是将Java层的方法名和本地函数对应起来，方便执行引擎在执行字节码时根据这些对应关系表来调用C/C++函数，如下面，将这些方法进行注册，执行引擎执行到hashCode方法时就可以通过关系表来查找到JVM的JVM_IHashCode函数，其中()I还可以得知Java层上的类型应该转为int类型。这个映射其实就可以看成将字符串映射到函数指针。

static JNINativeMethod methods[] = {

{"hashCode", "()I", (void *)&JVM_IHashCode},

{"wait", "(J)V", (void *)&JVM_MonitorWait},

{"notify", "()V", (void *)&JVM_MonitorNotify},

{"notifyAll", "()V", (void *)&JVM_MonitorNotifyAll},

{"clone", "()Ljava/lang/Object;", (void *)&JVM_Clone},

};

getClass方法

getClass方法也是个本地方法，对应的本地方法为Java_java_lang_Object_getClass，如下：

JNIEXPORT jclass JNICALLJava_java_lang_Object_getClass(JNIEnv *env, jobject this){

if (this == NULL) {

JNU_ThrowNullPointerException(env, NULL);

return 0;

} else {

return (*env)->GetObjectClass(env, this);

}

所以这里主要就是看GetObjectClass函数了，Java层的Class在C++层与之对应的则是klassOop，所以关于类的元数据和方法信息可以通过它获得。

JNI_ENTRY(jclass, jni_GetObjectClass(JNIEnv *env, jobject obj))

JNIWrapper("GetObjectClass");

DTRACE_PROBE2(hotspot_jni, GetObjectClass__entry, env, obj);

klassOop k = JNIHandles::resolve_non_null(obj)->klass();

jclass ret =

(jclass) JNIHandles::make_local(env, Klass::cast(k)->java_mirror());

DTRACE_PROBE1(hotspot_jni, GetObjectClass__return, ret);

return ret;

JNI_END

hashCode方法

由前面registerNatives方法将几个本地方法注册可知，hashCode方法对应的函数为JVM_IHashCode，即

JVM_ENTRY(jint, JVM_IHashCode(JNIEnv* env, jobject handle))

JVMWrapper("JVM_IHashCode");

// as implemented in the classic virtual machine; return 0 if object is NULL

return handle == NULL ? 0 : ObjectSynchronizer::FastHashCode (THREAD, JNIHandles::resolve_non_null(handle)) ;

JVM_END

对于hashcode生成的逻辑由synchronizer.cpp的get_next_hash函数决定，实现比较复杂，根据hashcode的不同值有不同的生成策略，最后使用一个hash掩码处理。

static inline intptr_t get_next_hash(Thread * Self, oop obj) {

intptr_t value = 0 ;

if (hashCode == 0) {

value = os::random() ;

} else

if (hashCode == 1) {

intptr_t addrBits = intptr_t(obj) >> 3 ;

value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;

} else

if (hashCode == 2) {

value = 1 ; // for sensitivity testing

} else

if (hashCode == 3) {

value = ++GVars.hcSequence ;

} else

if (hashCode == 4) {

value = intptr_t(obj) ;

} else {

unsigned t = Self->_hashStateX ;

t ^= (t << 11) ;

Self->_hashStateX = Self->_hashStateY ;

Self->_hashStateY = Self->_hashStateZ ;

Self->_hashStateZ = Self->_hashStateW ;

unsigned v = Self->_hashStateW ;

v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;

Self->_hashStateW = v ;

value = v ;

}

value &= markOopDesc::hash_mask;

if (value == 0) value = 0xBAD ;

assert (value != markOopDesc::no_hash, "invariant") ;

TEVENT (hashCode: GENERATE) ;

return value;

}

equals方法

这是一个非本地方法，判断逻辑也十分简单，直接==比较。

clone方法

由本地方法表知道clone方法对应的本地函数为JVM_Clone，clone方法主要实现对象的克隆功能，根据该对象生成一个相同的新对象（我们常见的类的对象的属性如果是原始类型则会克隆值，但如果是对象则会克隆对象的地址）。Java的类要实现克隆则需要实现Cloneable接口，if (!klass->is_cloneable())这里会校验是否有实现该接口。然后判断是否是数组分两种情况分配内存空间，新对象为new_obj，接着对new_obj进行copy及C++层数据结构的设置。最后再转成jobject类型方便转成Java层的Object类型。

JVM_ENTRY(jobject, JVM_Clone(JNIEnv* env, jobject handle))

JVMWrapper("JVM_Clone");

Handle obj(THREAD, JNIHandles::resolve_non_null(handle));

const KlassHandle klass (THREAD, obj->klass());

JvmtiVMObjectAllocEventCollector oam;

if (!klass->is_cloneable()) {

ResourceMark rm(THREAD);

THROW_MSG_0(vmSymbols::java_lang_CloneNotSupportedException(), klass->external_name());

}

const int size = obj->size();

oop new_obj = NULL;

if (obj->is_javaArray()) {

const int length = ((arrayOop)obj())->length();

new_obj = CollectedHeap::array_allocate(klass, size, length, CHECK_NULL);

} else {

new_obj = CollectedHeap::obj_allocate(klass, size, CHECK_NULL);

}

Copy::conjoint_jlongs_atomic((jlong*)obj(), (jlong*)new_obj,

(size_t)align_object_size(size) / HeapWordsPerLong);

new_obj->init_mark();

BarrierSet* bs = Universe::heap()->barrier_set();

assert(bs->has_write_region_opt(), "Barrier set does not have write_region");

bs->write_region(MemRegion((HeapWord*)new_obj, size));

if (klass->has_finalizer()) {

assert(obj->is_instance(), "should be instanceOop");

new_obj = instanceKlass::register_finalizer(instanceOop(new_obj), CHECK_NULL);

}

return JNIHandles::make_local(env, oop(new_obj));

JVM_END

toString方法

逻辑是获取class名称加上@再加上十六进制的hashCode。

notify方法

此方法用来唤醒线程，final修饰说明不可重写。与之对应的本地方法为JVM_MonitorNotify，ObjectSynchronizer::notify最终会调用ObjectMonitor::notify(TRAPS)，这个过程是ObjectSynchronizer会尝试当前线程获取free ObjectMonitor对象，不成功则尝试从全局中获取。

JVM_ENTRY(void, JVM_MonitorNotify(JNIEnv* env, jobject handle))

JVMWrapper("JVM_MonitorNotify");

Handle obj(THREAD, JNIHandles::resolve_non_null(handle));

assert(obj->is_instance() || obj->is_array(), "JVM_MonitorNotify must apply to an object");

ObjectSynchronizer::notify(obj, CHECK);

JVM_END

ObjectMonitor对象包含一个_WaitSet队列对象，此对象保存着所有处于wait状态的线程，用ObjectWaiter对象表示。notify要做的事是先获取_WaitSet队列锁，再取出_WaitSet队列中第一个ObjectWaiter对象，再根据不同策略处理该对象，比如把它加入到_EntryList队列中。然后再释放_WaitSet队列锁。它并没有释放synchronized对应的锁，所以锁只能等到synchronized同步块结束时才释放。

void ObjectMonitor::notify(TRAPS) {

CHECK_OWNER();

if (_WaitSet == NULL) {

TEVENT (Empty-Notify) ;

return ;

}

DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);

int Policy = Knob_MoveNotifyee ;

Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;

ObjectWaiter * iterator = DequeueWaiter() ;

if (iterator != NULL) {

TEVENT (Notify1 - Transfer) ;

guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;

guarantee (iterator->_notified == 0, "invariant") ;

if (Policy != 4) {

iterator->TState = ObjectWaiter::TS_ENTER ;

}

iterator->_notified = 1 ;

ObjectWaiter * List = _EntryList ;

if (List != NULL) {

assert (List->_prev == NULL, "invariant") ;

assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;

assert (List != iterator, "invariant") ;

}

if (Policy == 0) { // prepend to EntryList

if (List == NULL) {

iterator->_next = iterator->_prev = NULL ;

_EntryList = iterator ;

} else {

List->_prev = iterator ;

iterator->_next = List ;

iterator->_prev = NULL ;

_EntryList = iterator ;

}

} else

if (Policy == 1) { // append to EntryList

if (List == NULL) {

iterator->_next = iterator->_prev = NULL ;

_EntryList = iterator ;

} else {

// CONSIDER: finding the tail currently requires a linear-time walk of

// the EntryList. We can make tail access constant-time by converting to

// a CDLL instead of using our current DLL.

ObjectWaiter * Tail ;

for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;

assert (Tail != NULL && Tail->_next == NULL, "invariant") ;

Tail->_next = iterator ;

iterator->_prev = Tail ;

iterator->_next = NULL ;

}

} else

if (Policy == 2) { // prepend to cxq

// prepend to cxq

if (List == NULL) {

iterator->_next = iterator->_prev = NULL ;

_EntryList = iterator ;

} else {

iterator->TState = ObjectWaiter::TS_CXQ ;

for (;;) {

ObjectWaiter * Front = _cxq ;

iterator->_next = Front ;

if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {

break ;

}

} else

if (Policy == 3) { // append to cxq

iterator->TState = ObjectWaiter::TS_CXQ ;

for (;;) {

ObjectWaiter * Tail ;

Tail = _cxq ;

if (Tail == NULL) {

iterator->_next = NULL ;

if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {

break ;

}

} else {

while (Tail->_next != NULL) Tail = Tail->_next ;

Tail->_next = iterator ;

iterator->_prev = Tail ;

iterator->_next = NULL ;

break ;

}

} else {

ParkEvent * ev = iterator->_event ;

iterator->TState = ObjectWaiter::TS_RUN ;

OrderAccess::fence() ;

ev->unpark() ;

}

if (Policy < 4) {

iterator->wait_reenter_begin(this);

}

// _WaitSetLock protects the wait queue, not the EntryList. We could

// move the add-to-EntryList operation, above, outside the critical section

// protected by _WaitSetLock. In practice that's not useful. With the

// exception of wait() timeouts and interrupts the monitor owner

// is the only thread that grabs _WaitSetLock. There's almost no contention

// on _WaitSetLock so it's not profitable to reduce the length of the

// critical section.

}

Thread::SpinRelease (&_WaitSetLock) ;

if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {

ObjectMonitor::_sync_Notifications->inc() ;

}

notifyAll方法

与notify方法类似，只是在取_WaitSet队列时不是取第一个而是取所有。

wait方法

wait方法是让线程等待，它对应的本地方法是JVM_MonitorWait，间接调用了ObjectSynchronizer::wait，与notify对应，它也是对应调用ObjectMonitor对象的wait方法。该方法较长，这里不贴出来了，大概就是创建一个ObjectWaiter对象，接着获取_WaitSet队列锁将ObjectWaiter对象添加到该队列中，再释放队列锁。另外，它还会释放synchronized对应的锁，所以锁没有等到synchronized同步块结束时才释放。

JVM_ENTRY(void, JVM_MonitorWait(JNIEnv* env, jobject handle, jlong ms))

JVMWrapper("JVM_MonitorWait");

Handle obj(THREAD, JNIHandles::resolve_non_null(handle));

assert(obj->is_instance() || obj->is_array(), "JVM_MonitorWait must apply to an object");

JavaThreadInObjectWaitState jtiows(thread, ms != 0);

if (JvmtiExport::should_post_monitor_wait()) {

JvmtiExport::post_monitor_wait((JavaThread *)THREAD, (oop)obj(), ms);

}

ObjectSynchronizer::wait(obj, ms, CHECK);

JVM_END

finalize方法

这个方法用于当对象被回收时调用，这个由JVM支持，Object的finalize方法默认是什么都没有做，如果子类需要在对象被回收时执行一些逻辑处理，则可以重写finalize方法。