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【java源码系列】 - Object

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本文字数： 13k 阅读时长 ≈ 12 分钟

架构

  • Object.java(rt.jar) 调用 jdk的native方法(Object.c)。
  • native方法 调用hotspot jvm的方法(jvm.cpp)。

Object.java

  • java source版本：oracle 1.7

Object 还有隐形的构造函数.

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new Object();
new Object[int]; // 在ArrayList的构造函数中用到了

疑问：

为什么Object.java中看不到构造函数

编译器为它生成了构造函数

为什么在stack trace中看不到调用Object.

有7个native方法。

  • private static native void registerNatives();
  • protected native Object clone() throws CloneNotSupportedException;
  • public final native Class<?> getClass();
  • public native int hashCode();
  • public final native void notify();
  • public final native void notifyAll();
  • public final native void wait(long timeout) throws InterruptedException;
  • 但是构造函数是怎样实现的呢？(见jvm目录的init.md)

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，将生成一个java_lang_Object.h头文件(自动生成的header，没什么意义)

Object.c

  • source版本：openjdk-7-fcs-src-b147-27_jun_2011
  • 路径: openjdk\jdk\src\share\native\java\lang\Object.c
  • 没有Object.h

Object.c

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/*-
 *      Implementation of class Object
 *
 *      former threadruntime.c, Sun Sep 22 12:09:39 1991
 */

#include <stdio.h>
#include <signal.h>
#include <limits.h>

#include "jni.h"
#include "jni_util.h"
#include "jvm.h"

#include "java_lang_Object.h"

// JVM_这些函数是在jvm.c中实现的
static JNINativeMethod methods[] = {
    {"hashCode",    "()I",                    (void *)&JVM_IHashCode},  //  返回int
    {"wait",        "(J)V",                   (void *)&JVM_MonitorWait}, // 返回void，参数是long
    {"notify",      "()V",                    (void *)&JVM_MonitorNotify},  // 返回void
    {"notifyAll",   "()V",                    (void *)&JVM_MonitorNotifyAll}, // 返回void
    {"clone",       "()Ljava/lang/Object;",   (void *)&JVM_Clone}, // 返回Ojbect
};

JNIEXPORT void JNICALL
Java_java_lang_Object_registerNatives(JNIEnv *env, jclass cls)
{
    (*env)->RegisterNatives(env, cls,
                            methods, sizeof(methods)/sizeof(methods[0]));
}

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);
    }
}

其中JNINativeMethod的结构体如下:

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typedef struct {
    char *name; // Java中函数的名字
    char *signature; // signature 方法签名，描述了函数的参数和返回值
    void *fnPtr;  // native实现的函数指针，指向C函数
} JNINativeMethod;

跟踪 hashcode

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// jvm.h 路径: openjdk\hotspot\src\share\vm\prims\jvm.h
JNIEXPORT jint JNICALL
JVM_IHashCode(JNIEnv *env, jobject obj);


// jvm.cpp 路径: openjdk\hotspot\src\share\vm\prims\jvm.cpp
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

FastHashCode才是真正计算hashcode的代码

FastHashCode

这是hashCode()的具体实现

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// 路径: openjdk\hotspot\src\share\vm\runtime\synchronizer.cpp
intptr_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 对象的hashcode存储在对象头里
    if (hash) {                       // if it has hash, just return it 注意这里有个cache，对于同一个Ojbect，第一次调用Object.hashCode将会执行实际的计算并记入cache，以后直接从cache中取出。
      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();  // 取出缓存
  if (hash == 0) {
    hash = get_next_hash(Self, obj);  // 实际计算
    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.");
    }
  }
  // We finally get the hash
  return hash;
}

get_next_hash

这才是核心代码

又调用的get_next_hash()

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// 路径: openjdk\hotspot\src\share\vm\runtime\synchronizer.cpp
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;
}

hashCode()并不是简单的返回内存地址。
OpenJDK一共实现了5中不同的计算hash值的方法，通过
这段代码中hashCode进行切换。其中hashCode == 4的是直接使用地址的（前面的实验说明OpenJDK默认情况下并没有使用这种方式，或许可以通过运行/编译时参数进行选择）。

结论

前面通过JNI验证已经能够得到很显然的结论，hashCode返回的并不一定是对象的（虚拟）内存地址，具体取决于运行时库和JVM的具体实现。

跟踪wait

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JNIEXPORT void JNICALL
JVM_MonitorWait(JNIEnv *env, jobject obj, jlong ms);


// 路径: openjdk\hotspot\src\share\vm\prims\jvm.cpp
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

跟踪notify

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JNIEXPORT void JNICALL
JVM_MonitorNotify(JNIEnv *env, jobject obj);


// 路径: openjdk\hotspot\src\share\vm\prims\jvm.cpp
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

跟踪clone

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JNIEXPORT jobject JNICALL
JVM_Clone(JNIEnv *env, jobject obj);


// 路径: openjdk\hotspot\src\share\vm\prims\jvm.cpp
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;

跟踪 getClass

跟踪

参考

http://blog.csdn.net/xusiwei1236/article/details/45152201

byte code

  • 通过编译后的jar包(即class文件)，查看byte code.
  • 运行 javap -c java.lang.Object > a.txt，得到以下的byte code

如果执行不成功，看看是否把jdk的lib加入到了classpath:

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Compiled from "Object.java"
public class java.lang.Object {
  public java.lang.Object();  // 什么都不干？不需要调用<init>吗？
    Code:
       0: return        

  public final native java.lang.Class<?> getClass();

  public native int hashCode();

  public boolean equals(java.lang.Object);
    Code:
       0: aload_0       
       1: aload_1       
       2: if_acmpne     9
       5: iconst_1      
       6: goto          10
       9: iconst_0      
      10: ireturn       

  protected native java.lang.Object clone() throws java.lang.CloneNotSupportedException;

  public java.lang.String toString();
    Code:
       0: new           #1                  // class java/lang/StringBuilder
       3: dup           
       4: invokespecial #2                  // Method java/lang/StringBuilder."<init>":()V           // 注意这里，调用了<init>
       7: aload_0       
       8: invokevirtual #3                  // Method getClass:()Ljava/lang/Class;
      11: invokevirtual #4                  // Method java/lang/Class.getName:()Ljava/lang/String;
      14: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      17: ldc           #6                  // String @
      19: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      22: aload_0       
      23: invokevirtual #7                  // Method hashCode:()I
      26: invokestatic  #8                  // Method java/lang/Integer.toHexString:(I)Ljava/lang/String;
      29: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      32: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      35: areturn       

  public final native void notify();

  public final native void notifyAll();

  public final native void wait(long) throws java.lang.InterruptedException;

  public final void wait(long, int) throws java.lang.InterruptedException;
    Code:
       0: lload_1       
       1: lconst_0      
       2: lcmp          
       3: ifge          16
       6: new           #10                 // class java/lang/IllegalArgumentException
       9: dup           
      10: ldc           #11                 // String timeout value is negative
      12: invokespecial #12                 // Method java/lang/IllegalArgumentException."<init>":(Ljava/lang/String;)V
      15: athrow        
      16: iload_3       
      17: iflt          26
      20: iload_3       
      21: ldc           #13                 // int 999999
      23: if_icmple     36
      26: new           #10                 // class java/lang/IllegalArgumentException
      29: dup           
      30: ldc           #14                 // String nanosecond timeout value out of range
      32: invokespecial #12                 // Method java/lang/IllegalArgumentException."<init>":(Ljava/lang/String;)V
      35: athrow        
      36: iload_3       
      37: ldc           #15                 // int 500000
      39: if_icmpge     52
      42: iload_3       
      43: ifeq          56
      46: lload_1       
      47: lconst_0      
      48: lcmp          
      49: ifne          56
      52: lload_1       
      53: lconst_1      
      54: ladd          
      55: lstore_1      
      56: aload_0       
      57: lload_1       
      58: invokevirtual #16                 // Method wait:(J)V
      61: return        

  public final void wait() throws java.lang.InterruptedException;
    Code:
       0: aload_0       
       1: lconst_0      
       2: invokevirtual #16                 // Method wait:(J)V
       5: return        

  protected void finalize() throws java.lang.Throwable;
    Code:
       0: return        

  static {};
    Code:
       0: invokestatic  #17                 // Method registerNatives:()V
       3: return        
}

C:\Program Files\java\jdk1.7.0_67\jre\lib\rt>

其他

  • object header: it’s JVM dependent， 具体参考JVM

扩展阅读

  • Object.hashCode()的返回值到底是不是对象内存地址