2021-05-04 19:35  阅读(2662)
文章分类:从 JDK 源码看 Java 文章标签:JavaJDK 源码
©  原文作者:超人汪小建(seaboat) 原文地址:https://blog.csdn.net/wangyangzhizhou/column/info/16032

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getClasshashCodeequalsclonetoStringnotifywait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 JNICALL
        Java_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 JNICALL
        Java_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.cppget_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_MonitorNotifyObjectSynchronizer::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。

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