一、爲什麼要有Unsfae、我們爲什麼要了解這個類
1. java通常的代碼無法直接使用操作底層的硬件，爲了使java具備該能力，增加了Unsafe類
2.java的併發包中底層大量的使用這個類的功能，瞭解這個類有助於瞭解java 併發包，理解爲什麼併發包中的類的併發性能爲什麼會比使用synchronized好
二、Unsafe的主要功能
1.讀功能
讀功能有兩類：
1）讀內存中某一塊的數據，getXXXVolatile和getXXX。前者支持Volatile語義，後者不支持，可以看最後面的c代碼瞭解不同
2）獲取某一個類中某一個屬性在非堆內存中的位置,因爲在非堆中，這個屬性需要滿足下面兩個屬性之一：
a。這個屬性是一個靜態屬性
b。這個屬性是一個volatile修飾的屬性
2.寫功能
寫功能主要是三方面：putXXX,putOrderXXX,putXXXVolatile。
從性能上講，這三個方法由高到低；從數據的實時性講，這三個方法是由低到高。可以看最後面的c代碼瞭解不同
3. cas操作
這些方法類似compareAndSwapXXX，這些操作是一個原子操作，也就是說cpu在執行cas時對這一塊內存是獨佔排他的。在併發包中很多操作真正執行的也是cas，併發包中的類併發性能比使用 synchronized 關鍵字好也在於此：鎖的粒度小了許多並且少了線程上下文切換。
4.park、unpark
是併發包中鎖的基礎，將一個線程進行掛起是通過park方法實現的，調用 park後，線程將一直阻塞直到超時或者中斷等條件出現。unpark可以終止一個掛起的線程，使其恢復正常。
三、Unsafe類源碼
上面是對Unsafe類的小結，如果要對其有詳細的瞭解，需要看其源碼。
幾點說明：1.這個源代碼對應的jdk版本是有些老的 2.在jdk9中，這個類很有可能會去掉，儘管這個類被許多中間件用到了（http://www.linuxidc.com/Linux/2015-08/121833.htm）3.這個版本來自於http://www.cnblogs.com/mickole/articles/3757278.html
//下面是sun.misc.Unsafe.java類源碼 package sun.misc; import java.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. * 這個類提供了一個更底層的操作並且應該在受信任的代碼中使用。可以通過內存地址 * 存取fields,如果給出的內存地址是無效的那麼會有一個不確定的運行表現。 * * @author Tom Tromey ([email protected]) * @author Andrew John Hughes ([email protected]) */ public class Unsafe { // Singleton class. private static Unsafe unsafe = new Unsafe(); /*** * Private default constructor to prevent creation of an arbitrary * number of instances. * 使用私有默認構造器防止創建多個實例 */ private Unsafe() { } /*** * Retrieve the singleton instance of <code>Unsafe</code>. 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<code>Unsafe</code>的單例,這個方法調用應該防止在不可信的代碼中實例， * 因爲unsafe類提供了一個低級別的操作，例如直接內存存取。 * * @throws SecurityException 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屬性 */ public static Unsafe getUnsafe() { SecurityManager sm = System.getSecurityManager(); if (sm != null) sm.checkPropertiesAccess(); return unsafe; } /*** * 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field的內存地址偏移量,在這個類的其他方法中這個值只是被用作一個訪問 * 特定field的一個方式。這個值對於 給定的field是唯一的，並且後續對該方法的調用都應該 * 返回相同的值。 * * @param field the field whose offset should be returned. * 需要返回偏移量的field * @return the offset of the given field. * 指定field的偏移量 */ public native long objectFieldOffset(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。這個方法 * 的操作應該是原子的，因此提供了一種不可中斷的方式更新integer field。 * * @param obj the object containing the field to modify. * 包含要修改field的對象 * @param offset the offset of the integer field within <code>obj</code>. * <code>obj</code>中整型field的偏移量 * @param expect the expected value of the field. * 希望field中存在的值 * @param update the new value of the field if it equals <code>expect</code>. * 如果期望值expect與field的當前值相同，設置filed的值爲這個新值 * @return true if the field was changed. * 如果field的值被更改 */ public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update); /*** * 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。這個方法 * 的操作應該是原子的，因此提供了一種不可中斷的方式更新long field。 * * @param obj the object containing the field to modify. * 包含要修改field的對象 * @param offset the offset of the long field within <code>obj</code>. * <code>obj</code>中long型field的偏移量 * @param expect the expected value of the field. * 希望field中存在的值 * @param update the new value of the field if it equals <code>expect</code>. * 如果期望值expect與field的當前值相同，設置filed的值爲這個新值 * @return true if the field was changed. * 如果field的值被更改 */ public native boolean compareAndSwapLong(Object obj, long offset, long expect, long update); /*** * 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。這個方法 * 的操作應該是原子的，因此提供了一種不可中斷的方式更新object field。 * * @param obj the object containing the field to modify. * 包含要修改field的對象 * @param offset the offset of the object field within <code>obj</code>. * <code>obj</code>中object型field的偏移量 * @param expect the expected value of the field. * 希望field中存在的值 * @param update the new value of the field if it equals <code>expect</code>. * 如果期望值expect與field的當前值相同，設置filed的值爲這個新值 * @return true if the field was changed. * 如果field的值被更改 */ public native boolean compareAndSwapObject(Object obj, long offset, 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 <code>putIntVolatile(Object,long,int)</code>, which * doesn't guarantee the immediate visibility of the change to other * threads. It is only really useful where the integer field is * <code>volatile</code>, and is thus expected to change unexpectedly. * 設置obj對象中offset偏移地址對應的整型field的值爲指定值。這是一個有序或者 * 有延遲的<code>putIntVolatile</cdoe>方法，並且不保證值的改變被其他線程立 * 即看到。只有在field被<code>volatile</code>修飾並且期望被意外修改的時候 * 使用纔有用。 * * @param obj the object containing the field to modify. * 包含需要修改field的對象 * @param offset the offset of the integer field within <code>obj</code>. * <code>obj</code>中整型field的偏移量 * @param value the new value of the field. * field將被設置的新值 * @see #putIntVolatile(Object,long,int) */ public native void putOrderedInt(Object obj, long offset, int value); /*** * 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 <code>putLongVolatile(Object,long,long)</code>, which * doesn't guarantee the immediate visibility of the change to other * threads. It is only really useful where the long field is * <code>volatile</code>, and is thus expected to change unexpectedly. * 設置obj對象中offset偏移地址對應的long型field的值爲指定值。這是一個有序或者 * 有延遲的<code>putLongVolatile</cdoe>方法，並且不保證值的改變被其他線程立 * 即看到。只有在field被<code>volatile</code>修飾並且期望被意外修改的時候 * 使用纔有用。 * * @param obj the object containing the field to modify. * 包含需要修改field的對象 * @param offset the offset of the long field within <code>obj</code>. * <code>obj</code>中long型field的偏移量 * @param value the new value of the field. * field將被設置的新值 * @see #putLongVolatile(Object,long,long) */ public native void putOrderedLong(Object obj, long offset, long value); /*** * 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 <code>putObjectVolatile(Object,long,Object)</code>, which * doesn't guarantee the immediate visibility of the change to other * threads. It is only really useful where the object field is * <code>volatile</code>, and is thus expected to change unexpectedly. * 設置obj對象中offset偏移地址對應的object型field的值爲指定值。這是一個有序或者 * 有延遲的<code>putObjectVolatile</cdoe>方法，並且不保證值的改變被其他線程立 * 即看到。只有在field被<code>volatile</code>修飾並且期望被意外修改的時候 * 使用纔有用。 * * @param obj the object containing the field to modify. * 包含需要修改field的對象 * @param offset the offset of the object field within <code>obj</code>. * <code>obj</code>中long型field的偏移量 * @param value the new value of the field. * field將被設置的新值 */ public native void putOrderedObject(Object obj, long offset, 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. * 設置obj對象中offset偏移地址對應的整型field的值爲指定值。支持volatile store語義 * * @param obj the object containing the field to modify. * 包含需要修改field的對象 * @param offset the offset of the integer field within <code>obj</code>. * <code>obj</code>中整型field的偏移量 * @param value the new value of the field. * field將被設置的新值 */ public native void putIntVolatile(Object obj, long offset, int value); /*** * Retrieves the value of the integer field at the specified offset in the * supplied object with volatile load semantics. * 獲取obj對象中offset偏移地址對應的整型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的對象 * @param offset the offset of the integer field within <code>obj</code>. * <code>obj</code>中整型field的偏移量 */ public native int getIntVolatile(Object obj, long offset); /*** * Sets the value of the long field at the specified offset in the * supplied object to the given value, with volatile store semantics. * 設置obj對象中offset偏移地址對應的long型field的值爲指定值。支持volatile store語義 * * @param obj the object containing the field to modify. * 包含需要修改field的對象 * @param offset the offset of the long field within <code>obj</code>. * <code>obj</code>中long型field的偏移量 * @param value the new value of the field. * field將被設置的新值 * @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. * 設置obj對象中offset偏移地址對應的long型field的值爲指定值。 * * @param obj the object containing the field to modify. * 包含需要修改field的對象 * @param offset the offset of the long field within <code>obj</code>. * <code>obj</code>中long型field的偏移量 * @param value the new value of the field. * field將被設置的新值 * @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對象中offset偏移地址對應的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的對象 * @param offset the offset of the long field within <code>obj</code>. * <code>obj</code>中long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對象中offset偏移地址對應的long型field的值 * * @param obj the object containing the field to read. * 包含需要去讀取的field的對象 * @param offset the offset of the long field within <code>obj</code>. * <code>obj</code>中long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. * 設置obj對象中offset偏移地址對應的object型field的值爲指定值。支持volatile store語義 * * @param obj the object containing the field to modify. * 包含需要修改field的對象 * @param offset the offset of the object field within <code>obj</code>. * <code>obj</code>中object型field的偏移量 * @param value the new value of the field. * field將被設置的新值 * @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. * 設置obj對象中offset偏移地址對應的object型field的值爲指定值。 * * @param obj the object containing the field to modify. * 包含需要修改field的對象 * @param offset the offset of the object field within <code>obj</code>. * <code>obj</code>中object型field的偏移量 * @param value the new value of the field. * field將被設置的新值 * @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對象中offset偏移地址對應的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的對象 * @param offset the offset of the object field within <code>obj</code>. * <code>obj</code>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 * <a href="#arrayIndexScale"><code>arrayIndexScale</code></a>, * if non-zero. * 獲取給定數組中第一個元素的偏移地址。 * 爲了存取數組中的元素，這個偏移地址與<a href="#arrayIndexScale"><code>arrayIndexScale * </code></a>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. * 第一個元素地址被獲取的class * @return the offset of the first element of the array class. * 數組第一個元素 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 * <a href="#arrayBaseOffset"><code>arrayBaseOffset</code></a> * to access elements of the class. * 獲取用戶給定數組尋址的換算因子.一個合適的換算因子不能返回的時候(例如：基本類型), * 返回0.這個返回值能夠與<a href="#arrayBaseOffset"><code>arrayBaseOffset</code> * </a>一起使用去存取這個數組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 * <a href="#park"><code>park</code></a>. This method can also be used * to terminate a blockage caused by a prior call to <code>park</code>. * 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<a href="#park"><code>park</code></a>創建的在一個線程上的阻塞.這個 * 方法也可以被使用來終止一個先前調用<code>park</code>導致的阻塞. * 這個操作操作時不安全的,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 * <a href="#unpark"><code>unpark</code></a> occurs, the thread is * interrupted or the optional timeout expires. If an <code>unpark</code> * call has already occurred, this also counts. A timeout value of zero * is defined as no timeout. When <code>isAbsolute</code> is * <code>true</code>, 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). * 阻塞一個線程直到<a href="#unpark"><code>unpark</code></a>出現、線程 * 被中斷或者timeout時間到期。如果一個<code>unpark</code>調用已經出現了， * 這裏只計數。timeout爲0表示永不過期.當<code>isAbsolute</code>爲true時， * timeout是相對於新紀元之後的毫秒。否則這個值就是超時前的納秒數。這個方法執行時 * 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. * 如果爲true timeout的值是一個相對於新紀元之後的毫秒數 * @param time either the number of nanoseconds to wait, or a time in * milliseconds from the epoch to wait for. * 可以是一個要等待的納秒數，或者是一個相對於新紀元之後的毫秒數直到 * 到達這個時間點 */ public native void park(boolean isAbsolute, long time); }
natUnsafe.cc
// 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 <gcj/cni.h> #include <gcj/field.h> #include <gcj/javaprims.h> #include <jvm.h> #include <sun/misc/Unsafe.h> #include <java/lang/System.h> #include <java/lang/InterruptedException.h> #include <java/lang/Thread.h> #include <java/lang/Long.h> #include "sysdep/locks.h" // Use a spinlock for multi-word accesses class spinlock { 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 bool compareAndSwap (volatile jint *addr, jint old, jint new_val) { jboolean result = false; spinlock lock; if ((result = (*addr == old))) *addr = new_val; return result; } static inline bool compareAndSwap (volatile jlong *addr, jlong old, jlong new_val) { jboolean result = false; spinlock lock; if ((result = (*addr == old))) *addr = new_val; return result; } static inline bool compareAndSwap (volatile jobject *addr, jobject old, jobject new_val) { jboolean result = false; spinlock lock; if ((result = (*addr == old))) *addr = new_val; return result; } 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); return compareAndSwap (addr, expect, update); } jboolean sun::misc::Unsafe::compareAndSwapLong (jobject obj, jlong offset, jlong expect, jlong update) { volatile jlong *addr = (jlong*)((char *) obj + offset); return compareAndSwap (addr, expect, update); } jboolean sun::misc::Unsafe::compareAndSwapObject (jobject obj, jlong offset, jobject expect, jobject update) { jobject *addr = (jobject*)((char *) obj + offset); return compareAndSwap (addr, expect, update); } void sun::misc::Unsafe::putOrderedInt (jobject obj, jlong offset, jint value) { volatile jint *addr = (jint *) ((char *) obj + offset); *addr = value; } void sun::misc::Unsafe::putOrderedLong (jobject obj, jlong offset, jlong value) { volatile jlong *addr = (jlong *) ((char *) obj + offset); spinlock lock; *addr = value; } void sun::misc::Unsafe::putOrderedObject (jobject obj, jlong offset, jobject value) { volatile jobject *addr = (jobject *) ((char *) obj + offset); *addr = value; } void sun::misc::Unsafe::putIntVolatile (jobject obj, jlong offset, jint value) { write_barrier (); volatile jint *addr = (jint *) ((char *) obj + offset); *addr = value; } void sun::misc::Unsafe::putLongVolatile (jobject obj, jlong offset, jlong value) { volatile jlong *addr = (jlong *) ((char *) obj + offset); spinlock lock; *addr = value; } void sun::misc::Unsafe::putObjectVolatile (jobject obj, jlong offset, jobject value) { write_barrier (); volatile jobject *addr = (jobject *) ((char *) obj + offset); *addr = value; } #if 0 // FIXME void sun::misc::Unsafe::putInt (jobject obj, jlong offset, jint value) { jint *addr = (jint *) ((char *) obj + offset); *addr = value; } #endif void sun::misc::Unsafe::putLong (jobject obj, jlong offset, jlong value) { jlong *addr = (jlong *) ((char *) obj + offset); spinlock lock; *addr = value; } void sun::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); }