Java中如何打印对象内存地址？

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，这个另说

Frolan frolan = new Frolan();
System.out.println(frolan);// 输出结果
com.test.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，就是我们要的地址值？

我们通过源码来分析一波

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() ;} elseif (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 ;} elseif (hashCode == 2) {value = 1 ;            // for sensitivity testing} elseif (hashCode == 3) {value = ++GVars.hcSequence ;} elseif (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，返回随机数
hashCode=1，将oop的地址做位运算、异或运算得到的结果
hashCode=2，固定值1
hashCode=3，返回递增序列当前值
hashCode=4，oop的地址
hashCode=其他值，简单理解为移位寄存器，线程安全

我们设置JVM启动参数来模拟一下

// -XX:hashCode=2
for (int i = 0; i < 3; i++) {Frolan frolan = new Frolan();System.out.println(frolan.hashCode());
}// 输出结果
1
1
1

// -XX:hashCode=3
for (int i = 0; i < 3; i++) {Frolan frolan = new Frolan();System.out.println(frolan.hashCode());
}// 输出结果
714
715
716

我们模拟了hashCode=2和3的情况，其它情况没那么好验证，感兴趣的话，后续大家一起交流下~

那么我们不设置启动参数，默认值是什么？

java -XX:+PrintFlagsFinal -version | grep hashCodeintx hashCode                     = 5                     {product}
java version "1.8.0_101"
Java(TM) SE Runtime Environment (build 1.8.0_101-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.101-b13, mixed mode)

可以看到默认值是5，所以我们不设置的情况下，打印的并不是对象的内存地址

网上很多说，hashCode=4，这里就是对象的内存地址，这个是错的，这里拿到的只是oop的地址，System.identityHashCode()方法拿到的也是这个地址

如果想要获取真正的对象地址，可以使用Java 对象布局 ( JOL ) 工具

引入依赖

<dependency><groupId>org.openjdk.jol</groupId><artifactId>jol-core</artifactId><version>0.9</version>
</dependency>

Frolan frolan = new Frolan();
System.out.println(VM.current().addressOf(frolan));// 输出结果
31867940040