寫在前頭
本文將通過 jol 工具包和 OpenJDK 源碼來說明對象頭中 hashCode 的設置。
爲了不浪費大家的時間，先說結論：
- 對象創建完畢後，對象頭中的 hashCode 爲 0。
- 只有對象調用了從 Object 繼承下來的 hashCode 方法，HotSpot 纔會把對象 hashCode 寫入對象頭，否則不會寫入。
驗證
一、jol 輸出驗證
jol 全稱 Java Object Layout，可以打印對象在堆裏的真實佈局。
可以直接去 maven 倉庫搜索，得到的 maven 依賴如下：
<!-- https://mvnrepository.com/artifact/org.openjdk.jol/jol-core -->
<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.10</version>
</dependency>
我機器的 JVM 是 64 位的 HotSpot。
1.1 創建一個對象後，我們先直接輸出其對象佈局
驗證代碼：
import org.openjdk.jol.info.ClassLayout;
class Lear {
boolean state;
}
public class App {
public static void main( String[] args ) {
Lear lear = new Lear();
System.out.println(ClassLayout.parseInstance(lear));
}
}
輸出：
最後一列（VALUE），就是對象在內存中真實的樣子。給出了 16 進制、二進制 和 十進制 三種打印輸出。
其中，前 12 個字節就是對象頭。12 個字節中前 8 個是 mark word，後 4 個是對象元數據指針。對象的 hashCode 就放在 mark word 中。
再來看一下對象頭 mark word 中各個 bit 的含義：
我們的對象現在處於無鎖狀態，所以對應的 hashCode 爲 mark word 中間的 31 個比特。
從輸出中可以看出，現在對象的 hashCode 爲 0。（我的機器按小端模式存儲數據。）
1.2 調用從 Object 繼承下來的 hashCode 方法
驗證代碼：
import org.openjdk.jol.info.ClassLayout;
class Lear {
boolean state;
}
public class App {
public static void main( String[] args ) {
Lear lear = new Lear();
System.out.println("對象 hashCode："+lear.hashCode());
System.out.println(ClassLayout.parseInstance(lear).toPrintable());
}
}
輸出：
由於我的機器按小端模式存儲數據，所以對象的 mark word 應該讀作，
16 進制：0x00 00 00 01 4a e5 a5 01
二進制：0b00000000 00000000 00000000 0[0000001 01001010 11100101 10100101] 00000001
取出中間 31 位的 hashCode：0b0000001 01001010 11100101 10100101
對應的十進制就爲：21685669
1.3 調用自己重寫的 hashCode 方法
驗證代碼：
import org.openjdk.jol.info.ClassLayout;
class Lear {
boolean state;
@Override
public int hashCode() {
return 255;
}
}
public class App {
public static void main( String[] args ) {
Lear lear = new Lear();
System.out.println("對象 hashCode："+lear.hashCode());
System.out.println(ClassLayout.parseInstance(lear).toPrintable());
}
}
輸出：
可以看到對象的 hashCode 並未寫入對象頭。
二、OpenJDK 源碼驗證
OpenJDK 源碼鏈接：http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/3462d04401ba/src/share/native/java/lang/Object.c ，查看 Object.c 文件，可以看到 hashCode 的方法被註冊成由 JVM_IHashCode 方法指針來處理。
static JNINativeMethod methods[] = {
{"hashCode", "()I", (void *)&JVM_IHashCode},
{"wait", "(J)V", (void *)&JVM_MonitorWait},
{"notify", "()V", (void *)&JVM_MonitorNotify},
{"notifyAll", "()V", (void *)&JVM_MonitorNotifyAll},
{"clone", "()Ljava/lang/Object;", (void *)&JVM_Clone},
};
而 JVM_IHashCode 方法指針在 openjdk\hotspot\src\share\vm\prims\jvm.cpp 中定義爲：
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
從而得知，真正計算獲得 hashCode 的值是 ObjectSynchronizer::FastHashCode。
ObjectSynchronizer::FastHashCode 方法的實現
在 openjdk\hotspot\src\share\vm\runtime\synchronizer.cpp 找到其實現方法。
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
if (hash) { // if it has hash, just return it
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();
if (hash == 0) {
hash = get_next_hash(Self, obj);
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;
}
該方法表明，如果對象頭中的哈希值非零，就直接返回對象頭中設置的哈希值，如果爲零，則調用 get_next_hash 方法計算哈希值，並將哈希值寫入對象頭。