java unsafe park_Java中Unsafe类详解
http://www.cnblogs.com/mickole/articles/3757278.html
Java不能直接訪問操作系統(tǒng)底層，而是通過本地方法來訪問。Unsafe類提供了硬件級(jí)別的原子操作，主要提供了以下功能：
1、通過Unsafe類可以分配內(nèi)存，可以釋放內(nèi)存；
類中提供的3個(gè)本地方法allocateMemory、reallocateMemory、freeMemory分別用于分配內(nèi)存，擴(kuò)充內(nèi)存和釋放內(nèi)存，與C語言中的3個(gè)方法對(duì)應(yīng)。
public native long allocateMemory(longl);
public native long reallocateMemory(long l, longl1);
public native void freeMemory(long l);
2、可以定位對(duì)象某字段的內(nèi)存位置，也可以修改對(duì)象的字段值，即使它是私有的；
字段的定位：
JAVA中對(duì)象的字段的定位可能通過staticFieldOffset方法實(shí)現(xiàn)，該方法返回給定field的內(nèi)存地址偏移量，這個(gè)值對(duì)于給定的filed是唯一的且是固定不變的。
getIntVolatile方法獲取對(duì)象中offset偏移地址對(duì)應(yīng)的整型field的值,支持volatile load語義。
getLong方法獲取對(duì)象中offset偏移地址對(duì)應(yīng)的long型field的值
數(shù)組元素定位：
Unsafe類中有很多以BASE_OFFSET結(jié)尾的常量，比如ARRAY_INT_BASE_OFFSET，ARRAY_BYTE_BASE_OFFSET等，這些常量值是通過arrayBaseOffset方法得到的。arrayBaseOffset方法是一個(gè)本地方法，可以獲取數(shù)組第一個(gè)元素的偏移地址。Unsafe類中還有很多以INDEX_SCALE結(jié)尾的常量，比如 ARRAY_INT_INDEX_SCALE ， ARRAY_BYTE_INDEX_SCALE等，這些常量值是通過arrayIndexScale方法得到的。arrayIndexScale方法也是一個(gè)本地方法，可以獲取數(shù)組的轉(zhuǎn)換因子，也就是數(shù)組中元素的增量地址。將arrayBaseOffset與arrayIndexScale配合使用，可以定位數(shù)組中每個(gè)元素在內(nèi)存中的位置。
public final classUnsafe {
public static final intARRAY_INT_BASE_OFFSET;
public static final intARRAY_INT_INDEX_SCALE;
public native longstaticFieldOffset(Field field);
public native int getIntVolatile(Object obj, longl);
public native long getLong(Object obj, longl);
public native intarrayBaseOffset(Class class1);
public native intarrayIndexScale(Class class1);
static{
ARRAY_INT_BASE_OFFSET =theUnsafe.arrayBaseOffset([I);
ARRAY_INT_INDEX_SCALE =theUnsafe.arrayIndexScale([I);
}
}
3、掛起與恢復(fù)
將一個(gè)線程進(jìn)行掛起是通過park方法實(shí)現(xiàn)的，調(diào)用 park后，線程將一直阻塞直到超時(shí)或者中斷等條件出現(xiàn)。unpark可以終止一個(gè)掛起的線程，使其恢復(fù)正常。整個(gè)并發(fā)框架中對(duì)線程的掛起操作被封裝在 LockSupport類中，LockSupport類中有各種版本pack方法，但最終都調(diào)用了Unsafe.park()方法。
public classLockSupport {
public static voidunpark(Thread thread) {
if (thread != null)
unsafe.unpark(thread);
}
public static voidpark(Object blocker) {
Thread t =Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(false, 0L);
setBlocker(t, null);
}
public static void parkNanos(Object blocker, longnanos) {
if (nanos > 0) {
Thread t =Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(false, nanos);
setBlocker(t, null);
}
}
public static void parkUntil(Object blocker, longdeadline) {
Thread t =Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(true, deadline);
setBlocker(t, null);
}
public static voidpark() {
unsafe.park(false, 0L);
}
public static void parkNanos(longnanos) {
if (nanos > 0)
unsafe.park(false, nanos);
}
public static void parkUntil(longdeadline) {
unsafe.park(true, deadline);
}
}
4、CAS操作
是通過compareAndSwapXXX方法實(shí)現(xiàn)的
/*** 比較obj的offset處內(nèi)存位置中的值和期望的值，如果相同則更新。此更新是不可中斷的。
*
* @paramobj 需要更新的對(duì)象
* @paramoffset obj中整型field的偏移量
* @paramexpect 希望field中存在的值
* @paramupdate 如果期望值expect與field的當(dāng)前值相同，設(shè)置filed的值為這個(gè)新值
* @return如果field的值被更改返回true
*/
public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update);
CAS操作有3個(gè)操作數(shù)，內(nèi)存值M，預(yù)期值E，新值U，如果M==E，則將內(nèi)存值修改為B，否則啥都不做。
參考資料：
首先介紹一下什么是Compare And Swap(CAS)？簡(jiǎn)單的說就是比較并交換。
CAS 操作包含三個(gè)操作數(shù) —— 內(nèi)存位置(V)、預(yù)期原值(A)和新值(B)。如果內(nèi)存位置的值與預(yù)期原值相匹配，那么處理器會(huì)自動(dòng)將該位置值更新為新值。否則，處理器不做任何操作。無論哪種情況，它都會(huì)在 CAS 指令之前返回該位置的值。CAS 有效地說明了“我認(rèn)為位置 V 應(yīng)該包含值 A；如果包含該值，則將 B 放到這個(gè)位置；否則，不要更改該位置，只告訴我這個(gè)位置現(xiàn)在的值即可?！?Java并發(fā)包(java.util.concurrent)中大量使用了CAS操作,涉及到并發(fā)的地方都調(diào)用了sun.misc.Unsafe類方法進(jìn)行CAS操作。
在看一下volatile, Volatile修飾的成員變量在每次被線程訪問時(shí)，都強(qiáng)迫從共享內(nèi)存中重讀該成員變量的值。而且，當(dāng)成員變量發(fā)生變化時(shí)，強(qiáng)迫線程將變化值回寫到共享內(nèi)存。這樣在任何時(shí)刻，兩個(gè)不同的線程總是看到某個(gè)成員變量的值是相同的，更簡(jiǎn)單一點(diǎn)理解就是volatile修飾的變量值發(fā)生變化時(shí)對(duì)于另外的線程是可見的。
如何正確使用volatile可以參考下面這篇文章：
下面來看看java中具體的CAS操作類sun.misc.Unsafe。Unsafe類提供了硬件級(jí)別的原子操作，Java無法直接訪問到操作系統(tǒng)底層(如系統(tǒng)硬件等)，為此Java使用native方法來擴(kuò)展Java程序的功能。具體實(shí)現(xiàn)使用c++,詳見文件sun.misc.natUnsafe.cc();sun.misc包的源代碼可以在這里找到：
//下面是sun.misc.Unsafe.java類源碼
packagesun.misc;
importjava.lang.reflect.Field;
/***
* This class should provide access to low-level operations and its
* use should be limited to trusted code. Fields can be accessed using
* memory addresses, with undefined behaviour occurring if invalid memory
* addresses are given.
* 這個(gè)類提供了一個(gè)更底層的操作并且應(yīng)該在受信任的代碼中使用。可以通過內(nèi)存地址
* 存取fields,如果給出的內(nèi)存地址是無效的那么會(huì)有一個(gè)不確定的運(yùn)行表現(xiàn)。
*
* @authorTom Tromey (tromey@redhat.com)
* @authorAndrew John Hughes (gnu_andrew@member.fsf.org)
*/
public classUnsafe
{
//Singleton class.
private static Unsafe unsafe = newUnsafe();
/***
* Private default constructor to prevent creation of an arbitrary
* number of instances.
* 使用私有默認(rèn)構(gòu)造器防止創(chuàng)建多個(gè)實(shí)例
*/
privateUnsafe()
{
}
/***
* Retrieve the singleton instance of Unsafe. The calling
* method should guard this instance from untrusted code, as it provides
* access to low-level operations such as direct memory access.
* 獲取Unsafe的單例,這個(gè)方法調(diào)用應(yīng)該防止在不可信的代碼中實(shí)例，
* 因?yàn)閡nsafe類提供了一個(gè)低級(jí)別的操作，例如直接內(nèi)存存取。
*
* @throwsSecurityException if a security manager exists and prevents
* access to the system properties.
* 如果安全管理器不存在或者禁止訪問系統(tǒng)屬性
*/
public staticUnsafe getUnsafe()
{
SecurityManager sm =System.getSecurityManager();
if (sm != null)
sm.checkPropertiesAccess();
returnunsafe;
}
/***
* Returns the memory address offset of the given static field.
* The offset is merely used as a means to access a particular field
* in the other methods of this class. The value is unique to the given
* field and the same value should be returned on each subsequent call.
* 返回指定靜態(tài)field的內(nèi)存地址偏移量,在這個(gè)類的其他方法中這個(gè)值只是被用作一個(gè)訪問
* 特定field的一個(gè)方式。這個(gè)值對(duì)于 給定的field是唯一的，并且后續(xù)對(duì)該方法的調(diào)用都應(yīng)該
* 返回相同的值。
*
* @paramfield the field whose offset should be returned.
* 需要返回偏移量的field
* @returnthe offset of the given field.
* 指定field的偏移量
*/
public native longobjectFieldOffset(Field field);
/***
* Compares the value of the integer field at the specified offset
* in the supplied object with the given expected value, and updates
* it if they match. The operation of this method should be atomic,
* thus providing an uninterruptible way of updating an integer field.
* 在obj的offset位置比較integer field和期望的值，如果相同則更新。這個(gè)方法
* 的操作應(yīng)該是原子的，因此提供了一種不可中斷的方式更新integer field。
*
* @paramobj the object containing the field to modify.
* 包含要修改field的對(duì)象
* @paramoffset the offset of the integer field within obj.
* obj中整型field的偏移量
* @paramexpect the expected value of the field.
* 希望field中存在的值
* @paramupdate the new value of the field if it equals expect.
* 如果期望值expect與field的當(dāng)前值相同，設(shè)置filed的值為這個(gè)新值
* @returntrue if the field was changed.
* 如果field的值被更改
*/
public native boolean compareAndSwapInt(Object obj, longoffset,
int expect, intupdate);
/***
* Compares the value of the long field at the specified offset
* in the supplied object with the given expected value, and updates
* it if they match. The operation of this method should be atomic,
* thus providing an uninterruptible way of updating a long field.
* 在obj的offset位置比較long field和期望的值，如果相同則更新。這個(gè)方法
* 的操作應(yīng)該是原子的，因此提供了一種不可中斷的方式更新long field。
*
* @paramobj the object containing the field to modify.
* 包含要修改field的對(duì)象
* @paramoffset the offset of the long field within obj.
* obj中l(wèi)ong型field的偏移量
* @paramexpect the expected value of the field.
* 希望field中存在的值
* @paramupdate the new value of the field if it equals expect.
* 如果期望值expect與field的當(dāng)前值相同，設(shè)置filed的值為這個(gè)新值
* @returntrue if the field was changed.
* 如果field的值被更改
*/
public native boolean compareAndSwapLong(Object obj, longoffset,
long expect, longupdate);
/***
* Compares the value of the object field at the specified offset
* in the supplied object with the given expected value, and updates
* it if they match. The operation of this method should be atomic,
* thus providing an uninterruptible way of updating an object field.
* 在obj的offset位置比較object field和期望的值，如果相同則更新。這個(gè)方法
* 的操作應(yīng)該是原子的，因此提供了一種不可中斷的方式更新object field。
*
* @paramobj the object containing the field to modify.
* 包含要修改field的對(duì)象
* @paramoffset the offset of the object field within obj.
* obj中object型field的偏移量
* @paramexpect the expected value of the field.
* 希望field中存在的值
* @paramupdate the new value of the field if it equals expect.
* 如果期望值expect與field的當(dāng)前值相同，設(shè)置filed的值為這個(gè)新值
* @returntrue if the field was changed.
* 如果field的值被更改
*/
public native boolean compareAndSwapObject(Object obj, longoffset,
Object expect, Object update);
/***
* Sets the value of the integer field at the specified offset in the
* supplied object to the given value. This is an ordered or lazy
* version of putIntVolatile(Object,long,int), which
* doesn't guarantee the immediate visibility of the change to other
* threads. It is only really useful where the integer field is
* volatile, and is thus expected to change unexpectedly.
* 設(shè)置obj對(duì)象中offset偏移地址對(duì)應(yīng)的整型field的值為指定值。這是一個(gè)有序或者
* 有延遲的putIntVolatile方法，并且不保證值的改變被其他線程立
* 即看到。只有在field被volatile修飾并且期望被意外修改的時(shí)候
* 使用才有用。
*
* @paramobj the object containing the field to modify.
* 包含需要修改field的對(duì)象
* @paramoffset the offset of the integer field within obj.
* obj中整型field的偏移量
* @paramvalue the new value of the field.
* field將被設(shè)置的新值
* @see#putIntVolatile(Object,long,int)
*/
public native void putOrderedInt(Object obj, long offset, intvalue);
/***
* Sets the value of the long field at the specified offset in the
* supplied object to the given value. This is an ordered or lazy
* version of putLongVolatile(Object,long,long), which
* doesn't guarantee the immediate visibility of the change to other
* threads. It is only really useful where the long field is
* volatile, and is thus expected to change unexpectedly.
* 設(shè)置obj對(duì)象中offset偏移地址對(duì)應(yīng)的long型field的值為指定值。這是一個(gè)有序或者
* 有延遲的putLongVolatile方法，并且不保證值的改變被其他線程立
* 即看到。只有在field被volatile修飾并且期望被意外修改的時(shí)候
* 使用才有用。
*
* @paramobj the object containing the field to modify.
* 包含需要修改field的對(duì)象
* @paramoffset the offset of the long field within obj.
* obj中l(wèi)ong型field的偏移量
* @paramvalue the new value of the field.
* field將被設(shè)置的新值
* @see#putLongVolatile(Object,long,long)
*/
public native void putOrderedLong(Object obj, long offset, longvalue);
/***
* Sets the value of the object field at the specified offset in the
* supplied object to the given value. This is an ordered or lazy
* version of putObjectVolatile(Object,long,Object), which
* doesn't guarantee the immediate visibility of the change to other
* threads. It is only really useful where the object field is
* volatile, and is thus expected to change unexpectedly.
* 設(shè)置obj對(duì)象中offset偏移地址對(duì)應(yīng)的object型field的值為指定值。這是一個(gè)有序或者
* 有延遲的putObjectVolatile方法，并且不保證值的改變被其他線程立
* 即看到。只有在field被volatile修飾并且期望被意外修改的時(shí)候
* 使用才有用。
*
* @paramobj the object containing the field to modify.
* 包含需要修改field的對(duì)象
* @paramoffset the offset of the object field within obj.
* obj中l(wèi)ong型field的偏移量
* @paramvalue the new value of the field.
* field將被設(shè)置的新值
*/
public native void putOrderedObject(Object obj, longoffset, Object value);
/***
* Sets the value of the integer field at the specified offset in the
* supplied object to the given value, with volatile store semantics.
* 設(shè)置obj對(duì)象中offset偏移地址對(duì)應(yīng)的整型field的值為指定值。支持volatile store語義
*
* @paramobj the object containing the field to modify.
* 包含需要修改field的對(duì)象
* @paramoffset the offset of the integer field within obj.
* obj中整型field的偏移量
* @paramvalue the new value of the field.
* field將被設(shè)置的新值
*/
public native void putIntVolatile(Object obj, long offset, intvalue);
/***
* Retrieves the value of the integer field at the specified offset in the
* supplied object with volatile load semantics.
* 獲取obj對(duì)象中offset偏移地址對(duì)應(yīng)的整型field的值,支持volatile load語義。
*
* @paramobj the object containing the field to read.
* 包含需要去讀取的field的對(duì)象
* @paramoffset the offset of the integer field within obj.
* obj中整型field的偏移量
*/
public native int getIntVolatile(Object obj, longoffset);
/***
* Sets the value of the long field at the specified offset in the
* supplied object to the given value, with volatile store semantics.
* 設(shè)置obj對(duì)象中offset偏移地址對(duì)應(yīng)的long型field的值為指定值。支持volatile store語義
*
* @paramobj the object containing the field to modify.
* 包含需要修改field的對(duì)象
* @paramoffset the offset of the long field within obj.
* obj中l(wèi)ong型field的偏移量
* @paramvalue the new value of the field.
* field將被設(shè)置的新值
* @see#putLong(Object,long,long)
*/
public native void putLongVolatile(Object obj, long offset, long value);
/***
* Sets the value of the long field at the specified offset in the
* supplied object to the given value.
* 設(shè)置obj對(duì)象中offset偏移地址對(duì)應(yīng)的long型field的值為指定值。
*
* @param obj the object containing the field to modify.
* 包含需要修改field的對(duì)象
* @param offset the offset of the long field within obj.
* obj中l(wèi)ong型field的偏移量
* @param value the new value of the field.
* field將被設(shè)置的新值
* @see #putLongVolatile(Object,long,long)
*/
public native void putLong(Object obj, long offset, long value);
/***
* Retrieves the value of the long field at the specified offset in the
* supplied object with volatile load semantics.
* 獲取obj對(duì)象中offset偏移地址對(duì)應(yīng)的long型field的值,支持volatile load語義。
*
* @param obj the object containing the field to read.
* 包含需要去讀取的field的對(duì)象
* @param offset the offset of the long field within obj.
* obj中l(wèi)ong型field的偏移量
* @see #getLong(Object,long)
*/
public native long getLongVolatile(Object obj, long offset);
/***
* Retrieves the value of the long field at the specified offset in the
* supplied object.
* 獲取obj對(duì)象中offset偏移地址對(duì)應(yīng)的long型field的值
*
* @param obj the object containing the field to read.
* 包含需要去讀取的field的對(duì)象
* @param offset the offset of the long field within obj.
* obj中l(wèi)ong型field的偏移量
* @see #getLongVolatile(Object,long)
*/
public native long getLong(Object obj, long offset);
/***
* Sets the value of the object field at the specified offset in the
* supplied object to the given value, with volatile store semantics.
* 設(shè)置obj對(duì)象中offset偏移地址對(duì)應(yīng)的object型field的值為指定值。支持volatile store語義
*
* @param obj the object containing the field to modify.
* 包含需要修改field的對(duì)象
* @param offset the offset of the object field within obj.
* obj中object型field的偏移量
* @param value the new value of the field.
* field將被設(shè)置的新值
* @see #putObject(Object,long,Object)
*/
public native void putObjectVolatile(Object obj, long offset, Object value);
/***
* Sets the value of the object field at the specified offset in the
* supplied object to the given value.
* 設(shè)置obj對(duì)象中offset偏移地址對(duì)應(yīng)的object型field的值為指定值。
*
* @param obj the object containing the field to modify.
* 包含需要修改field的對(duì)象
* @param offset the offset of the object field within obj.
* obj中object型field的偏移量
* @param value the new value of the field.
* field將被設(shè)置的新值
* @see #putObjectVolatile(Object,long,Object)
*/
public native void putObject(Object obj, long offset, Object value);
/***
* Retrieves the value of the object field at the specified offset in the
* supplied object with volatile load semantics.
* 獲取obj對(duì)象中offset偏移地址對(duì)應(yīng)的object型field的值,支持volatile load語義。
*
* @param obj the object containing the field to read.
* 包含需要去讀取的field的對(duì)象
* @param offset the offset of the object field within obj.
* obj中object型field的偏移量
*/
public native Object getObjectVolatile(Object obj, long offset);
/***
* Returns the offset of the first element for a given array class.
* To access elements of the array class, this value may be used along with
* with that returned by
* arrayIndexScale,
* if non-zero.
* 獲取給定數(shù)組中第一個(gè)元素的偏移地址。
* 為了存取數(shù)組中的元素，這個(gè)偏移地址與arrayIndexScale
* 方法的非0返回值一起被使用。
* @param arrayClass the class for which the first element's address should
* be obtained.
* 第一個(gè)元素地址被獲取的class
* @return the offset of the first element of the array class.
* 數(shù)組第一個(gè)元素 的偏移地址
* @see arrayIndexScale(Class)
*/
public native int arrayBaseOffset(Class arrayClass);
/***
* Returns the scale factor used for addressing elements of the supplied
* array class. Where a suitable scale factor can not be returned (e.g.
* for primitive types), zero should be returned. The returned value
* can be used with
* arrayBaseOffset
* to access elements of the class.
* 獲取用戶給定數(shù)組尋址的換算因子.一個(gè)合適的換算因子不能返回的時(shí)候(例如：基本類型),
* 返回0.這個(gè)返回值能夠與arrayBaseOffset
* 一起使用去存取這個(gè)數(shù)組class中的元素
*
* @param arrayClass the class whose scale factor should be returned.
* @return the scale factor, or zero if not supported for this array class.
*/
public native int arrayIndexScale(Class arrayClass);
/***
* Releases the block on a thread created by
* park. This method can also be used
* to terminate a blockage caused by a prior call to park.
* This operation is unsafe, as the thread must be guaranteed to be
* live. This is true of Java, but not native code.
* 釋放被park創(chuàng)建的在一個(gè)線程上的阻塞.這個(gè)
* 方法也可以被使用來終止一個(gè)先前調(diào)用park導(dǎo)致的阻塞.
* 這個(gè)操作操作時(shí)不安全的,因此線程必須保證是活的.這是java代碼不是native代碼。
* @param thread the thread to unblock.
* 要解除阻塞的線程
*/
public native void unpark(Thread thread);
/***
* Blocks the thread until a matching
* unpark occurs, the thread is
* interrupted or the optional timeout expires. If an unpark
* call has already occurred, this also counts. A timeout value of zero
* is defined as no timeout. When isAbsolute is
* true, the timeout is in milliseconds relative to the
* epoch. Otherwise, the value is the number of nanoseconds which must
* occur before timeout. This call may also return spuriously (i.e.
* for no apparent reason).
* 阻塞一個(gè)線程直到unpark出現(xiàn)、線程
* 被中斷或者timeout時(shí)間到期。如果一個(gè)unpark調(diào)用已經(jīng)出現(xiàn)了，
* 這里只計(jì)數(shù)。timeout為0表示永不過期.當(dāng)isAbsolute為true時(shí)，
* timeout是相對(duì)于新紀(jì)元之后的毫秒。否則這個(gè)值就是超時(shí)前的納秒數(shù)。這個(gè)方法執(zhí)行時(shí)
* 也可能不合理地返回(沒有具體原因)
*
* @param isAbsolute true if the timeout is specified in milliseconds from
* the epoch.
* 如果為true timeout的值是一個(gè)相對(duì)于新紀(jì)元之后的毫秒數(shù)
* @param time either the number of nanoseconds to wait, or a time in
* milliseconds from the epoch to wait for.
* 可以是一個(gè)要等待的納秒數(shù)，或者是一個(gè)相對(duì)于新紀(jì)元之后的毫秒數(shù)直到
* 到達(dá)這個(gè)時(shí)間點(diǎn)
*/
public native void park(boolean isAbsolute, long time);
}
下面這個(gè)例子演示了簡(jiǎn)單的修改一個(gè)byte[]的數(shù)據(jù)。
這個(gè)例子在eclipse里不能直接編譯，要到項(xiàng)目的屬性，JavaCompiler，Errors/Warnings中Forbidden reference(access rules)中設(shè)置為warning。
另外，因?yàn)閟un.misc.Unsafe包不能直接使用，所有代碼里用反射的技巧得到了一個(gè)Unsafe的實(shí)例。
importjava.lang.reflect.Field;
importjava.util.Arrays;
importsun.misc.Unsafe;
public classTest {
private static intbyteArrayBaseOffset;
public static void main(String[] args) throwsSecurityException,
NoSuchFieldException, IllegalArgumentException,
IllegalAccessException {
Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
theUnsafe.setAccessible(true);
Unsafe UNSAFE = (Unsafe) theUnsafe.get(null);
System.out.println(UNSAFE);
byte[] data = new byte[10];
System.out.println(Arrays.toString(data));
byteArrayBaseOffset = UNSAFE.arrayBaseOffset(byte[].class);
System.out.println(byteArrayBaseOffset);
UNSAFE.putByte(data, byteArrayBaseOffset, (byte) 1);
UNSAFE.putByte(data, byteArrayBaseOffset + 5, (byte) 5);
System.out.println(Arrays.toString(data));
}
}
運(yùn)行結(jié)果：
sun.misc.Unsafe@6af62373
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
24[1, 0, 0, 0, 0, 5, 0, 0, 0, 0]
以下為Unsafe類涉及到的源碼：
Unsafe.java
/**Unsafe.java - Unsafe operations needed for concurrency
Copyright (C) 2006 Free Software Foundation
This file is part of GNU Classpath.
GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.
GNU Classpath is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License
along with GNU Classpath; see the file COPYING. If not, write to the
Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
02110-1301 USA.
Linking this library statically or dynamically with other modules is
making a combined work based on this library. Thus, the terms and
conditions of the GNU General Public License cover the whole
combination.
As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
executable, regardless of the license terms of these independent
modules, and to copy and distribute the resulting executable under
terms of your choice, provided that you also meet, for each linked
independent module, the terms and conditions of the license of that
module. An independent module is a module which is not derived from
or based on this library. If you modify this library, you may extend
this exception to your version of the library, but you are not
obligated to do so. If you do not wish to do so, delete this
exception statement from your version. */
packagesun.misc;
importjava.lang.reflect.Field;
/***
* This class should provide access to low-level operations and its
* use should be limited to trusted code. Fields can be accessed using
* memory addresses, with undefined behaviour occurring if invalid memory
* addresses are given.
*
* @authorTom Tromey (tromey@redhat.com)
* @authorAndrew John Hughes (gnu_andrew@member.fsf.org)
*/
public classUnsafe
{
//Singleton class.
private static Unsafe unsafe = newUnsafe();
/***
* Private default constructor to prevent creation of an arbitrary
* number of instances.
*/
privateUnsafe()
{
}
/***
* Retrieve the singleton instance of Unsafe. The calling
* method should guard this instance from untrusted code, as it provides
* access to low-level operations such as direct memory access.
*
* @throwsSecurityException if a security manager exists and prevents
* access to the system properties.
*/
public staticUnsafe getUnsafe()
{
SecurityManager sm =System.getSecurityManager();
if (sm != null)
sm.checkPropertiesAccess();
returnunsafe;
}
/***
* Returns the memory address offset of the given static field.
* The offset is merely used as a means to access a particular field
* in the other methods of this class. The value is unique to the given
* field and the same value should be returned on each subsequent call.
*
* @paramfield the field whose offset should be returned.
* @returnthe offset of the given field.
*/
public native longobjectFieldOffset(Field field);
/***
* Compares the value of the integer field at the specified offset
* in the supplied object with the given expected value, and updates
* it if they match. The operation of this method should be atomic,
* thus providing an uninterruptible way of updating an integer field.
*
* @paramobj the object containing the field to modify.
* @paramoffset the offset of the integer field within obj.
* @paramexpect the expected value of the field.
* @paramupdate the new value of the field if it equals expect.
* @returntrue if the field was changed.
*/
public native boolean compareAndSwapInt(Object obj, longoffset,
int expect, intupdate);
/***
* Compares the value of the long field at the specified offset
* in the supplied object with the given expected value, and updates
* it if they match. The operation of this method should be atomic,
* thus providing an uninterruptible way of updating a long field.
*
* @paramobj the object containing the field to modify.
* @paramoffset the offset of the long field within obj.
* @paramexpect the expected value of the field.
* @paramupdate the new value of the field if it equals expect.
* @returntrue if the field was changed.
*/
public native boolean compareAndSwapLong(Object obj, longoffset,
long expect, longupdate);
/***
* Compares the value of the object field at the specified offset
* in the supplied object with the given expected value, and updates
* it if they match. The operation of this method should be atomic,
* thus providing an uninterruptible way of updating an object field.
*
* @paramobj the object containing the field to modify.
* @paramoffset the offset of the object field within obj.
* @paramexpect the expected value of the field.
* @paramupdate the new value of the field if it equals expect.
* @returntrue if the field was changed.
*/
public native boolean compareAndSwapObject(Object obj, longoffset,
Object expect, Object update);
/***
* Sets the value of the integer field at the specified offset in the
* supplied object to the given value. This is an ordered or lazy
* version of putIntVolatile(Object,long,int), which
* doesn't guarantee the immediate visibility of the change to other
* threads. It is only really useful where the integer field is
* volatile, and is thus expected to change unexpectedly.
*
* @paramobj the object containing the field to modify.
* @paramoffset the offset of the integer field within obj.
* @paramvalue the new value of the field.
* @see#putIntVolatile(Object,long,int)
*/
public native void putOrderedInt(Object obj, long offset, intvalue);
/***
* Sets the value of the long field at the specified offset in the
* supplied object to the given value. This is an ordered or lazy
* version of putLongVolatile(Object,long,long), which
* doesn't guarantee the immediate visibility of the change to other
* threads. It is only really useful where the long field is
* volatile, and is thus expected to change unexpectedly.
*
* @paramobj the object containing the field to modify.
* @paramoffset the offset of the long field within obj.
* @paramvalue the new value of the field.
* @see#putLongVolatile(Object,long,long)
*/
public native void putOrderedLong(Object obj, long offset, longvalue);
/***
* Sets the value of the object field at the specified offset in the
* supplied object to the given value. This is an ordered or lazy
* version of putObjectVolatile(Object,long,Object), which
* doesn't guarantee the immediate visibility of the change to other
* threads. It is only really useful where the object field is
* volatile, and is thus expected to change unexpectedly.
*
* @paramobj the object containing the field to modify.
* @paramoffset the offset of the object field within obj.
* @paramvalue the new value of the field.
*/
public native void putOrderedObject(Object obj, longoffset, Object value);
/***
* Sets the value of the integer field at the specified offset in the
* supplied object to the given value, with volatile store semantics.
*
* @paramobj the object containing the field to modify.
* @paramoffset the offset of the integer field within obj.
* @paramvalue the new value of the field.
*/
public native void putIntVolatile(Object obj, long offset, intvalue);
/***
* Retrieves the value of the integer field at the specified offset in the
* supplied object with volatile load semantics.
*
* @paramobj the object containing the field to read.
* @paramoffset the offset of the integer field within obj.
*/
public native int getIntVolatile(Object obj, longoffset);
/***
* Sets the value of the long field at the specified offset in the
* supplied object to the given value, with volatile store semantics.
*
* @paramobj the object containing the field to modify.
* @paramoffset the offset of the long field within obj.
* @param value the new value of the field.
* @see #putLong(Object,long,long)
*/
public native void putLongVolatile(Object obj, long offset, long value);
/***
* Sets the value of the long field at the specified offset in the
* supplied object to the given value.
*
* @param obj the object containing the field to modify.
* @param offset the offset of the long field within obj.
* @param value the new value of the field.
* @see #putLongVolatile(Object,long,long)
*/
public native void putLong(Object obj, long offset, long value);
/***
* Retrieves the value of the long field at the specified offset in the
* supplied object with volatile load semantics.
*
* @param obj the object containing the field to read.
* @param offset the offset of the long field within obj.
* @see #getLong(Object,long)
*/
public native long getLongVolatile(Object obj, long offset);
/***
* Retrieves the value of the long field at the specified offset in the
* supplied object.
*
* @param obj the object containing the field to read.
* @param offset the offset of the long field within obj.
* @see #getLongVolatile(Object,long)
*/
public native long getLong(Object obj, long offset);
/***
* Sets the value of the object field at the specified offset in the
* supplied object to the given value, with volatile store semantics.
*
* @param obj the object containing the field to modify.
* @param offset the offset of the object field within obj.
* @param value the new value of the field.
* @see #putObject(Object,long,Object)
*/
public native void putObjectVolatile(Object obj, long offset, Object value);
/***
* Sets the value of the object field at the specified offset in the
* supplied object to the given value.
*
* @param obj the object containing the field to modify.
* @param offset the offset of the object field within obj.
* @param value the new value of the field.
* @see #putObjectVolatile(Object,long,Object)
*/
public native void putObject(Object obj, long offset, Object value);
/***
* Retrieves the value of the object field at the specified offset in the
* supplied object with volatile load semantics.
*
* @param obj the object containing the field to read.
* @param offset the offset of the object field within obj.
*/
public native Object getObjectVolatile(Object obj, long offset);
/***
* Returns the offset of the first element for a given array class.
* To access elements of the array class, this value may be used along
* with that returned by
* arrayIndexScale,
* if non-zero.
*
* @param arrayClass the class for which the first element's address should
* be obtained.
* @return the offset of the first element of the array class.
* @see arrayIndexScale(Class)
*/
public native int arrayBaseOffset(Class arrayClass);
/***
* Returns the scale factor used for addressing elements of the supplied
* array class. Where a suitable scale factor can not be returned (e.g.
* for primitive types), zero should be returned. The returned value
* can be used with
* arrayBaseOffset
* to access elements of the class.
*
* @param arrayClass the class whose scale factor should be returned.
* @return the scale factor, or zero if not supported for this array class.
*/
public native int arrayIndexScale(Class arrayClass);
/***
* Releases the block on a thread created by
* park. This method can also be used
* to terminate a blockage caused by a prior call to park.
* This operation is unsafe, as the thread must be guaranteed to be
* live. This is true of Java, but not native code.
*
* @param thread the thread to unblock.
*/
public native void unpark(Thread thread);
/***
* Blocks the thread until a matching
* unpark occurs, the thread is
* interrupted or the optional timeout expires. If an unpark
* call has already occurred, this also counts. A timeout value of zero
* is defined as no timeout. When isAbsolute is
* true, the timeout is in milliseconds relative to the
* epoch. Otherwise, the value is the number of nanoseconds which must
* occur before timeout. This call may also return spuriously (i.e.
* for no apparent reason).
*
* @param isAbsolute true if the timeout is specified in milliseconds from
* the epoch.
* @param time either the number of nanoseconds to wait, or a time in
* milliseconds from the epoch to wait for.
*/
public native void park(boolean isAbsolute, long time);
}
//DO NOT EDIT THIS FILE - it is machine generated -*- c++ -*-
#ifndef __sun_misc_Unsafe__
#define __sun_misc_Unsafe__
#pragma interface#include
extern "Java"{
namespacesun
{
namespacemisc
{
classUnsafe;
}
}
}
class sun::misc::Unsafe : public::java::lang::Object
{
Unsafe();
public:
static ::sun::misc::Unsafe *getUnsafe();
virtual jlong objectFieldOffset(::java::lang::reflect::Field *);
virtual jboolean compareAndSwapInt(::java::lang::Object *, jlong, jint, jint);
virtual jboolean compareAndSwapLong(::java::lang::Object *, jlong, jlong, jlong);
virtual jboolean compareAndSwapObject(::java::lang::Object *, jlong, ::java::lang::Object *, ::java::lang::Object *);
virtual void putOrderedInt(::java::lang::Object *, jlong, jint);
virtual void putOrderedLong(::java::lang::Object *, jlong, jlong);
virtual void putOrderedObject(::java::lang::Object *, jlong, ::java::lang::Object *);
virtual void putIntVolatile(::java::lang::Object *, jlong, jint);
virtual jint getIntVolatile(::java::lang::Object *, jlong);
virtual void putLongVolatile(::java::lang::Object *, jlong, jlong);
virtual void putLong(::java::lang::Object *, jlong, jlong);
virtual jlong getLongVolatile(::java::lang::Object *, jlong);
virtual jlong getLong(::java::lang::Object *, jlong);
virtual void putObjectVolatile(::java::lang::Object *, jlong, ::java::lang::Object *);
virtual void putObject(::java::lang::Object *, jlong, ::java::lang::Object *);
virtual ::java::lang::Object * getObjectVolatile(::java::lang::Object *, jlong);
virtual jint arrayBaseOffset(::java::lang::Class *);
virtual jint arrayIndexScale(::java::lang::Class *);
virtual void unpark(::java::lang::Thread *);
virtual voidpark(jboolean, jlong);
private:
static ::sun::misc::Unsafe * unsafe;
public:
static ::java::lang::Class class$;
};
#endif //__sun_misc_Unsafe__
//DO NOT EDIT THIS FILE - it is machine generated -*- c++ -*-
#ifndef __sun_misc_ServiceConfigurationError__
#define __sun_misc_ServiceConfigurationError__
#pragma interface#include
extern "Java"{
namespacesun
{
namespacemisc
{
classServiceConfigurationError;
}
}
}
class sun::misc::ServiceConfigurationError : public::java::lang::Error
{
public:
ServiceConfigurationError();
ServiceConfigurationError(::java::lang::String *);
static ::java::lang::Class class$;
};
#endif //__sun_misc_ServiceConfigurationError__
//DO NOT EDIT THIS FILE - it is machine generated -*- c++ -*-
#ifndef __sun_misc_Service__
#define __sun_misc_Service__
#pragma interface#include
extern "Java"{
namespacesun
{
namespacemisc
{
classService;
}
}
}
class sun::misc::Service : public::java::lang::Object
{
public:
Service();
static ::java::util::Iterator * providers(::java::lang::Class *, ::java::lang::ClassLoader *);
static ::java::lang::Class class$;
};
#endif //__sun_misc_Service__
//natUnsafe.cc - Implementation of sun.misc.Unsafe native methods.
/** Copyright (C) 2006, 2007
Free Software Foundation
This file is part of libgcj.
This software is copyrighted work licensed under the terms of the
Libgcj License. Please consult the file "LIBGCJ_LICENSE" for
details. */#include #include #include #include #include #include #include #include #include #include "sysdep/locks.h"
//Use a spinlock for multi-word accesses
classspinlock
{
static volatile obj_addr_t lock;
public:
spinlock ()
{
while (! compare_and_swap (&lock, 0, 1))
_Jv_ThreadYield ();
}
~spinlock ()
{
release_set (&lock, 0);
}
};
//This is a single lock that is used for all synchronized accesses if
//the compiler can't generate inline compare-and-swap operations. In
//most cases it'll never be used, but the i386 needs it for 64-bit
//locked accesses and so does PPC32. It's worth building libgcj with
//target=i486 (or above) to get the inlines.
volatile obj_addr_t spinlock::lock;
static inline boolcompareAndSwap (volatile jint *addr, jint old, jint new_val)
{
jboolean result = false;
spinlock lock;
if ((result = (*addr ==old)))
*addr =new_val;
returnresult;
}
static inline boolcompareAndSwap (volatile jlong *addr, jlong old, jlong new_val)
{
jboolean result = false;
spinlock lock;
if ((result = (*addr ==old)))
*addr =new_val;
returnresult;
}
static inline boolcompareAndSwap (volatile jobject *addr, jobject old, jobject new_val)
{
jboolean result = false;
spinlock lock;
if ((result = (*addr ==old)))
*addr =new_val;
returnresult;
}
jlong
sun::misc::Unsafe::objectFieldOffset (::java::lang::reflect::Field *field)
{
_Jv_Field *fld =_Jv_FromReflectedField (field);
//FIXME: what if it is not an instance field?
return fld->getOffset();
}
jint
sun::misc::Unsafe::arrayBaseOffset (jclass arrayClass)
{
//FIXME: assert that arrayClass is array.
jclass eltClass = arrayClass->getComponentType();
return(jint)(jlong) _Jv_GetArrayElementFromElementType (NULL, eltClass);
}
jint
sun::misc::Unsafe::arrayIndexScale (jclass arrayClass)
{
//FIXME: assert that arrayClass is array.
jclass eltClass = arrayClass->getComponentType();
if (eltClass->isPrimitive())
return eltClass->size();
return sizeof (void *);
}
//These methods are used when the compiler fails to generate inline
//versions of the compare-and-swap primitives.
jboolean
sun::misc::Unsafe::compareAndSwapInt (jobject obj, jlong offset,
jint expect, jint update)
{
jint *addr = (jint *)((char *)obj +offset);
returncompareAndSwap (addr, expect, update);
}
jboolean
sun::misc::Unsafe::compareAndSwapLong (jobject obj, jlong offset,
jlong expect, jlong update)
{
volatile jlong *addr = (jlong*)((char *) obj +offset);
returncompareAndSwap (addr, expect, update);
}
jboolean
sun::misc::Unsafe::compareAndSwapObject (jobject obj, jlong offset,
jobject expect, jobject update)
{
jobject *addr = (jobject*)((char *) obj +offset);
returncompareAndSwap (addr, expect, update);
}
voidsun::misc::Unsafe::putOrderedInt (jobject obj, jlong offset, jint value)
{
volatile jint *addr = (jint *) ((char *) obj +offset);
*addr =value;
}
voidsun::misc::Unsafe::putOrderedLong (jobject obj, jlong offset, jlong value)
{
volatile jlong *addr = (jlong *) ((char *) obj +offset);
spinlock lock;
*addr =value;
}
voidsun::misc::Unsafe::putOrderedObject (jobject obj, jlong offset, jobject value)
{
volatile jobject *addr = (jobject *) ((char *) obj +offset);
*addr =value;
}
voidsun::misc::Unsafe::putIntVolatile (jobject obj, jlong offset, jint value)
{
write_barrier ();
volatile jint *addr = (jint *) ((char *) obj +offset);
*addr =value;
}
voidsun::misc::Unsafe::putLongVolatile (jobject obj, jlong offset, jlong value)
{
volatile jlong *addr = (jlong *) ((char *) obj +offset);
spinlock lock;
*addr =value;
}
voidsun::misc::Unsafe::putObjectVolatile (jobject obj, jlong offset, jobject value)
{
write_barrier ();
volatile jobject *addr = (jobject *) ((char *) obj +offset);
*addr =value;
}
#if 0 //FIXME
voidsun::misc::Unsafe::putInt (jobject obj, jlong offset, jint value)
{
jint *addr = (jint *) ((char *) obj +offset);
*addr =value;
}
#endif
voidsun::misc::Unsafe::putLong (jobject obj, jlong offset, jlong value)
{
jlong *addr = (jlong *) ((char *) obj +offset);
spinlock lock;
*addr =value;
}
voidsun::misc::Unsafe::putObject (jobject obj, jlong offset, jobject value)
{
jobject *addr = (jobject *) ((char *) obj +offset);
*addr =value;
}
jint
sun::misc::Unsafe::getIntVolatile (jobject obj, jlong offset)
{
volatile jint *addr = (jint *) ((char *) obj +offset);
jint result = *addr;
read_barrier ();
return result;
}
jobject
sun::misc::Unsafe::getObjectVolatile (jobject obj, jlong offset)
{
volatile jobject *addr = (jobject *) ((char *) obj + offset);
jobject result = *addr;
read_barrier ();
return result;
}
jlong
sun::misc::Unsafe::getLong (jobject obj, jlong offset)
{
jlong *addr = (jlong *) ((char *) obj + offset);
spinlock lock;
return *addr;
}
jlong
sun::misc::Unsafe::getLongVolatile (jobject obj, jlong offset)
{
volatile jlong *addr = (jlong *) ((char *) obj + offset);
spinlock lock;
return *addr;
}
void
sun::misc::Unsafe::unpark (::java::lang::Thread *thread)
{
natThread *nt = (natThread *) thread->data;
nt->park_helper.unpark ();
}
void
sun::misc::Unsafe::park (jboolean isAbsolute, jlong time)
{
using namespace ::java::lang;
Thread *thread = Thread::currentThread();
natThread *nt = (natThread *) thread->data;
nt->park_helper.park (isAbsolute, time);
}
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