System.out.println(new Student());我知道输出的是引用,引用不是内存地址是吗,Student@de6ced,引用的后几位是什么啊如何算的啊,谢谢
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return getClass().getName() + "@" + Integer.toHexString(hashCode());
String.hashCode() 算法有点不一样,忘记具体了。
public int hashCode() {
int h = hash;
if (h == 0) {
int off = offset;
char val[] = value;
int len = count; for (int i = 0; i < len; i++) {
h = 31*h + val[off++];
}
hash = h;
}
return h;
}
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->()->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->()->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;
Oop temp, test;
intptr_t hash;
Oop = ReadStableMark (obj); // object should remain ineligible for biased locking
assert (!->has_bias_pattern(), "invariant") ;
if (->is_neutral()) {
hash = ->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 = ->copy_set_hash(hash); // merge the hash code into header
// use (machine word version) atomic operation to install the hash
test = (Oop) Atomic::cmpxchg_ptr(temp, obj->_addr(), );
if (test == ) {
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 (->has_monitor()) {
monitor = ->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)->locker())) {
temp = ->displaced__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
= monitor->header();
assert (->is_neutral(), "invariant") ;
hash = ->hash();
if (hash == 0) {
hash = get_next_hash(Self, obj);
temp = ->copy_set_hash(hash); // merge hash code into header
assert (temp->is_neutral(), "invariant") ;
test = (Oop) Atomic::cmpxchg_ptr(temp, monitor, );
if (test != ) {
// 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;
}这个函数基本上就在这了,原始的 hashCode 在 jdk/src/share/native/java/lang/Object.c 这个文件中,调来调去就调到上面那个函数去了。OpenJDK 6 的下载页面:http://download.java.net/openjdk/jdk6/ 上面有个 46.9MB 的 tar.gz 文件,下载回来后是整个 JDK(JVM、JDK 工具和 J2SE 类库和底层类库)的源代码,解压后有 254MB,大约有 28750 个文件。