Object是所有類的父類，也就是說(shuō)java中所有的類都是直接或者間接繼承自O(shè)bject類。比如你隨便創(chuàng)建一個(gè)classA，雖然沒(méi)有明說(shuō)，但默認(rèn)是extendsObject的。
后面的三個(gè)點(diǎn)"..."表示可以接受若干不確定數(shù)量的參數(shù)。老的寫法是Objectargs[]這樣，但新版本的java中推薦使用...來(lái)表示。例如
publicvoidgetSomething(String...strings)(){}
object是java中所有類的父類，也就是說(shuō)所有的類，不管是自己創(chuàng)建的類還是系統(tǒng)中的類都繼承自object類,也就是說(shuō)所有的類在任何場(chǎng)合都可以代替object類，根據(jù)里氏替換原則，子類在任何場(chǎng)合都可以代替其父類，而父類卻不一定能代替其子類,java中常說(shuō)的萬(wàn)物皆對(duì)象說(shuō)的其實(shí)就是這個(gè)道理!object類體現(xiàn)了oop思想中的多態(tài)，繼承，封裝，抽象四大特性！
object類是所有類的基類，不是數(shù)據(jù)類型。這個(gè)你可以查詢jdk文檔了解，所有類都繼承自O(shè)bject。
Object...objects這種參數(shù)定義是在不確定方法參數(shù)的情況下的一種多態(tài)表現(xiàn)形式。即這個(gè)方法可以傳遞多個(gè)參數(shù)，這個(gè)參數(shù)的個(gè)數(shù)是不確定的。這樣你在方法體中需要相應(yīng)的做些處理。因?yàn)镺bject是基類，所以使用Object...objects這樣的參數(shù)形式，允許一切繼承自O(shè)bject的對(duì)象作為參數(shù)。這種方法在實(shí)際中應(yīng)該還是比較少用的。
Object[]obj這樣的形式，就是一個(gè)Object數(shù)組構(gòu)成的參數(shù)形式。說(shuō)明這個(gè)方法的參數(shù)是固定的，是一個(gè)Object數(shù)組，至于這個(gè)數(shù)組中存儲(chǔ)的元素，可以是繼承自O(shè)bject的所有類的對(duì)象。
這些基礎(chǔ)東西建議你多看幾遍"Thinkinjava"
Java的Object是所有其他類的父類，從繼承的層次來(lái)看它就是最頂層根，所以它也是唯一一個(gè)沒(méi)有父類的類。它包含了對(duì)象常用的一些方法，比如getClass、hashCode、equals、clone、toString、notify、wait等常用方法。所以其他類繼承了Object后就可以不用重復(fù)實(shí)現(xiàn)這些方法。這些方法大多數(shù)是native方法，下面具體分析。
主要的代碼如下：
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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塊修飾，所以在加載Object類時(shí)就會(huì)執(zhí)行該方法，對(duì)應(yīng)的本地方法為Java_java_lang_Object_registerNatives，如下，
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JNIEXPORT void JNICALLJava_java_lang_Object_registerNatives(JNIEnv *env, jclass cls){ (*env)->RegisterNatives(env, cls, methods, sizeof(methods)/sizeof(methods[0]));} |
可以看到它間接調(diào)用了JNINativeInterface_結(jié)構(gòu)體的方法，簡(jiǎn)單可以看成是這樣：它干的事大概就是將Java層的方法名和本地函數(shù)對(duì)應(yīng)起來(lái)，方便執(zhí)行引擎在執(zhí)行字節(jié)碼時(shí)根據(jù)這些對(duì)應(yīng)關(guān)系表來(lái)調(diào)用C/C++函數(shù)，如下面，將這些方法進(jìn)行注冊(cè)，執(zhí)行引擎執(zhí)行到hashCode方法時(shí)就可以通過(guò)關(guān)系表來(lái)查找到JVM的JVM_IHashCode函數(shù)，其中()I還可以得知Java層上的類型應(yīng)該轉(zhuǎn)為int類型。這個(gè)映射其實(shí)就可以看成將字符串映射到函數(shù)指針。
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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方法也是個(gè)本地方法，對(duì)應(yīng)的本地方法為Java_java_lang_Object_getClass，如下：
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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); }} |
所以這里主要就是看GetObjectClass函數(shù)了，Java層的Class在C++層與之對(duì)應(yīng)的則是klassOop，所以關(guān)于類的元數(shù)據(jù)和方法信息可以通過(guò)它獲得。
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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方法將幾個(gè)本地方法注冊(cè)可知，hashCode方法對(duì)應(yīng)的函數(shù)為JVM_IHashCode，即
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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 |
對(duì)于hashcode生成的邏輯由synchronizer.cpp的get_next_hash函數(shù)決定，實(shí)現(xiàn)比較復(fù)雜，根據(jù)hashcode的不同值有不同的生成策略，最后使用一個(gè)hash掩碼處理。
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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
這是一個(gè)非本地方法，判斷邏輯也十分簡(jiǎn)單，直接==比較。
clone方法
由本地方法表知道clone方法對(duì)應(yīng)的本地函數(shù)為JVM_Clone，clone方法主要實(shí)現(xiàn)對(duì)象的克隆功能，根據(jù)該對(duì)象生成一個(gè)相同的新對(duì)象（我們常見(jiàn)的類的對(duì)象的屬性如果是原始類型則會(huì)克隆值，但如果是對(duì)象則會(huì)克隆對(duì)象的地址）。Java的類要實(shí)現(xiàn)克隆則需要實(shí)現(xiàn)Cloneable接口，if (!klass->is_cloneable())這里會(huì)校驗(yàn)是否有實(shí)現(xiàn)該接口。然后判斷是否是數(shù)組分兩種情況分配內(nèi)存空間，新對(duì)象為new_obj，接著對(duì)new_obj進(jìn)行copy及C++層數(shù)據(jù)結(jié)構(gòu)的設(shè)置。最后再轉(zhuǎn)成jobject類型方便轉(zhuǎn)成Java層的Object類型。
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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
邏輯是獲取class名稱加上@再加上十六進(jìn)制的hashCode。
notify方法
此方法用來(lái)喚醒線程，final修飾說(shuō)明不可重寫。與之對(duì)應(yīng)的本地方法為JVM_MonitorNotify，ObjectSynchronizer::notify最終會(huì)調(diào)用ObjectMonitor::notify(TRAPS)，這個(gè)過(guò)程是ObjectSynchronizer會(huì)嘗試當(dāng)前線程獲取freeObjectMonitor對(duì)象，不成功則嘗試從全局中獲取。
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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對(duì)象包含一個(gè)_WaitSet隊(duì)列對(duì)象，此對(duì)象保存著所有處于wait狀態(tài)的線程，用ObjectWaiter對(duì)象表示。notify要做的事是先獲取_WaitSet隊(duì)列鎖，再取出_WaitSet隊(duì)列中第一個(gè)ObjectWaiter對(duì)象，再根據(jù)不同策略處理該對(duì)象，比如把它加入到_EntryList隊(duì)列中。然后再釋放_(tái)WaitSet隊(duì)列鎖。它并沒(méi)有釋放synchronized對(duì)應(yīng)的鎖，所以鎖只能等到synchronized同步塊結(jié)束時(shí)才釋放。
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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
與notify方法類似，只是在取_WaitSet隊(duì)列時(shí)不是取第一個(gè)而是取所有。
wait方法
wait方法是讓線程等待，它對(duì)應(yīng)的本地方法是JVM_MonitorWait，間接調(diào)用了ObjectSynchronizer::wait，與notify對(duì)應(yīng)，它也是對(duì)應(yīng)調(diào)用ObjectMonitor對(duì)象的wait方法。該方法較長(zhǎng)，這里不貼出來(lái)了，大概就是創(chuàng)建一個(gè)ObjectWaiter對(duì)象，接著獲取_WaitSet隊(duì)列鎖將ObjectWaiter對(duì)象添加到該隊(duì)列中，再釋放隊(duì)列鎖。另外，它還會(huì)釋放synchronized對(duì)應(yīng)的鎖，所以鎖沒(méi)有等到synchronized同步塊結(jié)束時(shí)才釋放。
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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
這個(gè)方法用于當(dāng)對(duì)象被回收時(shí)調(diào)用，這個(gè)由JVM支持，Object的finalize方法默認(rèn)是什么都沒(méi)有做，如果子類需要在對(duì)象被回收時(shí)執(zhí)行一些邏輯處理，則可以重寫finalize方法
總結(jié)
以上就是本文關(guān)于Java從JDK源碼角度對(duì)Object進(jìn)行實(shí)例分析的全部?jī)?nèi)容，希望對(duì)大家有所幫助。感興趣的朋友可以繼續(xù)參閱本站其他相關(guān)專題，如有不足之處，歡迎留言指出。感謝朋友們對(duì)本站的支持！
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