java打印对象内存地址
打印一个对象，默认会调用toString()方法
public class Main {
public static void main(String[] args) {
Main m1 = new Main();
Main m2 = new Main();
System.out.println(m1);
System.out.println(m2);
}
}
会输出:
testing.Main@1b6d3586
testing.Main@4554617c
如果不自定义toString方法，会默认调用Object类toString()方法
Object类toString()方法：
public String toString() {
return this.getClass().getName() + "@" + Integer.toHexString(this.hashCode());
}
而hashCode是Object类中的native方法：
public native int hashCode();
JDK 8 Hotspot中，hashCode实现
https://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/a3f01f9da231/src/share/vm/runtime/synchronizer.cpp
关键代码get_next_hash()方法定义：
// 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 = 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;
}
由全局变量 hashCode取值，对应不同的生成策略
它定义在：
https://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/a3f01f9da231/src/share/vm/runtime/globals.hpp
product(intx, hashCode, 5, \
"(Unstable) select hashCode generation algorithm")
默认为5，非稳定参数（Unstable)可以通过-XX启动参数指定。
查看系统JVM参数设定：
$ java -XX:+PrintFlagsFinal -version | grep hashCode
intx hashCode = 5 {product}
java version "1.8.0_231"
Java(TM) SE Runtime Environment (build 1.8.0_231-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.231-b11, mixed mode)
尝试设置VM参数：
-XX:hashCode=2
再次运行得到结果：
testing.Main@1
testing.Main@1
当重写了对象的hashCode方法时
如果重写了hashCode()方法，再打印对象内存地址，使用System.identityHashCode()方法：
public class Main {
public static void main(String[] args) {
Main m1 = new Main();
Main m2 = new Main();
System.out.println(m1); // 5
System.out.println(m2); // 5
System.out.println(System.identityHashCode(m1)); // 对象内存地址 460141958
System.out.println(System.identityHashCode(m2)); // 对象内存地址 1163157884
}
@Override
public int hashCode() {
return 5;
}
}
当然，如果指定 -XX:hashCode=2输出结果还是为：
testing.Main@5
testing.Main@5
1
1
总结：
是否指定了VM参数
如果没有重写hashCode()方法，直接打印对象
重写了hashCode()方法，使用System.identityHashCode()打印内存地址
VM参数hashCode取值，参考：
https://juejin.cn/post/6971946031764209678