Java中如何打印对象内存地址？

标签：Java Self 打印 value hashCode Frolan frolan 内存地址

先看一个简单的程序，一般我们打印对象，大部分是下面的情况，可能会重写下toString()方法

public static void main(String[] args) {
	Frolan frolan = new Frolan();
	System.out.println(frolan);
}

打印结果

com.msedu.wisdomquestion.admin.entity.Frolan@2b80d80f

这个结果其实是调用了Object.toString打印出来的，就是类路径名+@+hashCode()的16进制数

public String toString() {
    return getClass().getName() + "@" + Integer.toHexString(hashCode());
}

这里的hashCode()是一个native方法，其实就是我们要的地址值，当然，这是有前提条件的，我们通过源码来看一下

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 = cast_from_oop<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 = cast_from_oop<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;
}

大概意思就是，会根据不同的hashCode返回不同的结果
hashCode=0，随机数
hashCode=1，对象内存地址做位运算、异或运算后的值
hashCode=2，固定值1
hashCode=3，返回递增序列数值
hashCode=4，返回对象内存地址
hashCode=其他值，简单理解为移位寄存器，线程安全

我们来模拟一下，设置JVM启动参数，-XX:hashCode=2

// -XX:hashCode=2
public static void main(String[] args) {
	for (int i = 0; i < 3; i++) {
		Frolan frolan = new Frolan();
		System.out.println(frolan.hashCode());
	}
}

打印结果

1
1
1

我们不设置启动参数，默认就是返回对象的内存地址，但打印出来一定是内存地址？
这个不一定，如果我们重写了hashCode()方法，那就不是了

那如果我们重写了hashCode()方法，要想打印对象的内存地址，这时候可以怎么搞？
可以用System.identityHashCode()方法

@Override
public int hashCode() {
	return Objects.hashCode(id, name);
}

public static void main(String[] args) {
	Frolan frolan = new Frolan();
	System.out.println(frolan);
	System.out.println(System.identityHashCode(frolan));
}

打印结果

Frolan{id=null, name='null'}
729864207

标签：Java,Self,打印,value,hashCode,Frolan,frolan,内存地址
From： https://www.cnblogs.com/huozhonghun/p/17119446.html