我们知道在java中可以直接打印字符串等内容，但是直接将对象进行打印输出时，并不会输出具体值

而是一些奇怪的东西，如创建一个学生对象并打印学生对象结果如下：

输出结果为： Student@16d3586，为什么会输出这么一个结果呢？

通过查看java API我们可以得知，Java直接输出一个类的对象的时候，会自动调用这个类的toString方法

这个方法在位于object类中的，而在Java中所有的类都继承Object类，所以所有的类都有toString方法。

通过源码我们发现该方法的实现是这样的:

getClass().getName()+'@'+Integer.toHexString(hashCode())

输出的结果是：

类所在的包名.类名 + @ + 哈希码值

我们尝试获取一个对象的哈希值，打印结果如下

那么hashcode() 获取的哈希码又是什么东西呢？

哈希码表示的是JVM虚拟机为这个Object对象分配的一个int类型的数值

JVM会使用对象的hashcode值来提高对HashMap、Hashtable哈希表存取对象的使用效率

具体如何生成的呢，？紧接着我们进入源码去具体分析：

首先通过以下地址下载源码：

openJDK 7 下载地址1： 。

获取源码以后进入openjdk\jdk\src\share\classes\java\lang 目录下

可以看到 Object.java源码，打开，查看hashCode()的定义如下所示：

public native int hashCode();

native 表示该方法是一个本地方法，Java将调用本地方法库对此方法的实现。

由于Object类中有JNI方法调用，按照JNI的规则，应当生成JNI 的头文件

在此目录下执行 javah -jni java.lang.Object 指令

将生成一java_lang_Object.h头文件

java_lang_Object.h头文件关于hashcode方法的信息如下所示：

/** Class:     java_lang_Object* Method:    hashCode* Signature: ()I*/JNIEXPORT jint JNICALL Java_java_lang_Object_hashCode (JNIEnv *, jobject);

然后我们再 打开 openjdk\jdk\src\share\native\java\lang\ 目录

查看Object.c文件，可以看到hashCode() 的方法被注册成有 JVM_IHashCode方法指针来处理：

#include #include#include#include "jni.h"#include "jni_util.h"#include "jvm.h"#include "java_lang_Object.h"static JNINativeMethod methods[] = {   {"hashCode",    "()I",                    (void *)&JVM_IHashCode}, //hashcode的方法指针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},};JNIEXPORT void JNICALLJava_java_lang_Object_registerNatives(JNIEnv *env, jclass cls){   (*env)->RegisterNatives(env, cls,                           methods, sizeof(methods)/sizeof(methods[0]));}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);   }}

JVM_IHashCode方法指针在 openjdk\hotspot\src\share\vm\prims\jvm.cpp 中定义，

如下：

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

如上可以看出，JVM_IHashCode方法中调用了ObjectSynchronizer::FastHashCode方法

ObjectSynchronizer::fashHashCode()方法在

openjdk\hotspot\src\share\vm\runtime\synchronizer.cpp 文件中实现，其核心代码实现如下所示：

// hashCode() generation ://// Possibilities:// * MD5Digest of {obj,stwRandom}// * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.// * A DES- or AES-style SBox[] mechanism// * One of the Phi-based schemes, such as://   2654435761 = 2^32 * Phi (golden ratio)//   HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;// * A variation of Marsaglia's shift-xor RNG scheme.// * (obj ^ stwRandom) is appealing, but can result//   in undesirable regularity in the hashCode values of adjacent objects//   (objects allocated back-to-back, in particular).  This could potentially//   result in hashtable collisions and reduced hashtable efficiency.//   There are simple ways to "diffuse" the middle address bits over the//   generated hashCode values://static inline intptr_t get_next_hash(Thread * Self, oop obj) { intptr_t value = 0 ; if (hashCode == 0) {    // This form uses an unguarded global Park-Miller RNG,    // so it's possible for two threads to race and generate the same RNG.    // On MP system we'll have lots of RW access to a global, so the    // mechanism induces lots of coherency traffic.    value = os::random() ; } else if (hashCode == 1) {    // This variation has the property of being stable (idempotent)    // between STW operations.  This can be useful in some of the 1-0    // synchronization schemes.    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 {    // Marsaglia's xor-shift scheme with thread-specific state    // This is probably the best overall implementation -- we'll    // likely make this the default in future releases.    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;}//   ObjectSynchronizer::FastHashCode方法的实现，该方法最终会返回我们期望已久的hashcodeintptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) { if (UseBiasedLocking) {   // NOTE: many places throughout the JVM do not expect a safepoint   // to be taken here, in particular most operations on perm gen   // objects. However, we only ever bias Java instances and all of   // the call sites of identity_hash that might revoke biases have   // been checked to make sure they can handle a safepoint. The   // added check of the bias pattern is to avoid useless calls to   // thread-local storage.   if (obj->mark()->has_bias_pattern()) {     // Box and unbox the raw reference just in case we cause a STW safepoint.     Handle hobj (Self, obj) ;     // Relaxing assertion for bug 6320749.     assert (Universe::verify_in_progress() ||             !SafepointSynchronize::is_at_safepoint(),            "biases should not be seen by VM thread here");     BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());     obj = hobj() ;     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");   } } // hashCode() is a heap mutator ... // Relaxing assertion for bug 6320749. assert (Universe::verify_in_progress() ||         !SafepointSynchronize::is_at_safepoint(), "invariant") ; assert (Universe::verify_in_progress() ||         Self->is_Java_thread() , "invariant") ; assert (Universe::verify_in_progress() ||        ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ; ObjectMonitor* monitor = NULL; markOop temp, test; intptr_t hash; markOop mark = ReadStableMark (obj); // object should remain ineligible for biased locking assert (!mark->has_bias_pattern(), "invariant") ; if (mark->is_neutral()) {   hash = mark->hash();              // this is a normal header   if (hash) {                       // if it has hash, just return it     return hash;   }   hash = get_next_hash(Self, obj);  // allocate a new hash code   temp = mark->copy_set_hash(hash); // merge the hash code into header   // use (machine word version) atomic operation to install the hash   test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);   if (test == mark) {     return hash;   }   // If atomic operation failed, we must inflate the header   // into heavy weight monitor. We could add more code here   // for fast path, but it does not worth the complexity. } else if (mark->has_monitor()) {   monitor = mark->monitor();   temp = monitor->header();   assert (temp->is_neutral(), "invariant") ;   hash = temp->hash();   if (hash) {     return hash;   }   // Skip to the following code to reduce code size } else if (Self->is_lock_owned((address)mark->locker())) {   temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned   assert (temp->is_neutral(), "invariant") ;   hash = temp->hash();              // by current thread, check if the displaced   if (hash) {                       // header contains hash code     return hash;   }   // WARNING:   //   The displaced header is strictly immutable.   // It can NOT be changed in ANY cases. So we have   // to inflate the header into heavyweight monitor   // even the current thread owns the lock. The reason   // is the BasicLock (stack slot) will be asynchronously   // read by other threads during the inflate() function.   // Any change to stack may not propagate to other threads   // correctly. } // Inflate the monitor to set hash code monitor = ObjectSynchronizer::inflate(Self, obj); // Load displaced header and check it has hash code mark = monitor->header(); assert (mark->is_neutral(), "invariant") ; hash = mark->hash(); if (hash == 0) {   hash = get_next_hash(Self, obj);   temp = mark->copy_set_hash(hash); // merge hash code into header   assert (temp->is_neutral(), "invariant") ;   test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);   if (test != mark) {     // The only update to the header in the monitor (outside GC)     // is install the hash code. If someone add new usage of     // displaced header, please update this code     hash = test->hash();     assert (test->is_neutral(), "invariant") ;     assert (hash != 0, "Trivial unexpected object/monitor header usage.");   } } return hash;}

通过如此复杂的步骤我们终于获取到哈希码值。

而我们在实际开发中，很多时候我们并不想打印对象的哈希码值

而是想获取该对象的具体属性，这个时候该怎么做呢？

我们已知直接打印对象时会自动调用toString方法

该方法是继承于Object类，所以为了能改变内容

很显然我们需要进行toString方法的重写，如下：

@Overridepublic String toString() {   return "Student{" +           "id=" + id +           ", name='" + name + '\'' +           ", age=" + age +           '}';}

重写以后发现我们打印内容不再是哈希码值，而是替换成我们自己想要获取的属性值