[转载]关于native,transient,volatile,synchronized四个关键字

http://www.blogjava.net/bacoo/archive/2007/12/20/169172.html

[转载]关于native,transient,volatile,synchronized四个关键字的使用
native
当你需要调用本地程序的时候
transient
你的类实现了java.io.Serializable而你又不想保存某个字段的时候
volatile
这个字段会被其他线程(直接或者间接)访问到，而你想保证每个线程都能得到最新的数据
（性能上肯定有损耗的，为了安全牺牲性能的事情多着去了）
------------------------------------------------------------------------------------
3，volatile
原CSDN有一篇介绍过，现贴回来。
volatile修饰变量。在每次被线程访问时，都强迫从共享内存中重读该成员变量的值。而且，当成员变量发生变化时，强迫线程将变化值回写到共享内存。这样在任何时刻，两个不同的线程总是看到某个成员变量的同一个值。 
看看Java Language Specification中的例子。 
条件：一个线程不停的调用方法one()，一个线程不停的调用方法two()。我测试过多次，这种情况好像一直没有出现。 
代码
class Test {   
    static int i = 0, j = 0;   
    static void one() { i++; j++; }   
    static void two() {   
        System.out.println("i=" + i + " j=" + j);   
    }   
}   
结果偶尔会出现j大于i的情况，因为方法没有同步，所以会出现i和j可能不是一次更新。一种防止这种情况发生的办法就是声明两个方法为synchronized 的。 
代码
class Test {   
    static int i = 0, j = 0;   
    static synchronized void one() { i++; j++; }   
    static synchronized void two() {   
        System.out.println("i=" + i + " j=" + j);   
    }   
}   
这样可以防止两个方法同时被执行，还可以保证j和i被同时更新，这样一来i和j的值一直是一样的。 
另外一种途径就是把i和j声明为volatile。 
代码
class Test {   
    static volatile int i = 0, j = 0;   
    static void one() { i++; j++; }   
    static void two() {   
        System.out.println("i=" + i + " j=" + j);   
    }   
}  
------------------------------------------------------------------------------------
2，transient
transient是一个变量修饰符,标记为transient的变量，在对一个对象进行序列化时，这些变量状态不会被序列化。
例如，假设某个类的成员变量是transient，那么当通过ObjectOutputStream把这个类的某个实例保存到磁盘上时，实际上transient变量的值是不会保存的。
当对象序列化的保存在存储器上时，不希望有些字段数据被保存，为了保证安全性，可以把这些字段声明为transient。
要更明白，可以看一些序列化的内容。
------------------------------------------------------------------------------------
1，native是方法修饰符。Native方法是由另外一种语言（如c/c++，FORTRAN，汇编）实现的本地方法。一般用于JNI中。
public class testdll 
{ 
static 
{ 
System.loadLibrary("test"); 
} 
    public native static int get(); 
    public native static void set(int i); 
    public static void main(String[] args) 
    { 
testdll test = new testdll(); 
test.set(10); 
System.out.println(test.get()); 
} 
} 
test.dll可以用VC写，例如，
-----testdll.h-----
/* DO NOT EDIT THIS FILE - it is machine generated */
#include  <jni.h >
/* Header for class testdll */
#ifndef _Included_testdll
#define _Included_testdll
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class:     testdll
* Method:    get
* Signature: ()I
*/
JNIEXPORT jint JNICALL Java_testdll_get
  (JNIEnv *, jclass);
/*
* Class:     testdll
* Method:    set
* Signature: (I)V
*/
JNIEXPORT void JNICALL Java_testdll_set
  (JNIEnv *, jclass, jint);
#ifdef __cplusplus
}
#endif
#endif
-------testdll.c-----------
#include "testdll.h" 
int i = 0; 
JNIEXPORT jint JNICALL Java_testdll_get (JNIEnv *, jclass) 
{ 
return i; 
} 
JNIEXPORT void JNICALL Java_testdll_set (JNIEnv *, jclass, jint j) 
{ 
i = j; 
}
------------------------------------------------------------------------------------


恐怕比较一下volatile和synchronized的不同是最容易解释清楚的。volatile是变量修饰符，而synchronized则作用于一段代码或方法；看如下三句get代码：

int i1;              int geti1() {return i1;}
volatile int i2;  int geti2() {return i2;}
int i3;              synchronized int geti3() {return i3;}
　　geti1()得到存储在当前线程中i1的数值。多个线程有多个i1变量拷贝，而且这些i1之间可以互不相同。换句话说，另一个线程可能已经改变了它线程内的i1值，而这个值可以和当前线程中的i1值不相同。事实上，Java有个思想叫“主”内存区域，这里存放了变量目前的“准确值”。每个线程可以有它自己的变量拷贝，而这个变量拷贝值可以和“主”内存区域里存放的不同。因此实际上存在一种可能：“主”内存区域里的i1值是1，线程1里的i1值是2，线程2里的i1值是3——这在线程1和线程2都改变了它们各自的i1值，而且这个改变还没来得及传递给“主”内存区域或其他线程时就会发生。
　　而geti2()得到的是“主”内存区域的i2数值。用volatile修饰后的变量不允许有不同于“主”内存区域的变量拷贝。换句话说，一个变量经volatile修饰后在所有线程中必须是同步的；任何线程中改变了它的值，所有其他线程立即获取到了相同的值。理所当然的，volatile修饰的变量存取时比一般变量消耗的资源要多一点，因为线程有它自己的变量拷贝更为高效。
　　既然volatile关键字已经实现了线程间数据同步，又要synchronized干什么呢？呵呵，它们之间有两点不同。首先，synchronized获得并释放监视器——如果两个线程使用了同一个对象锁，监视器能强制保证代码块同时只被一个线程所执行——这是众所周知的事实。但是，synchronized也同步内存：事实上，synchronized在“主”内存区域同步整个线程的内存。因此，执行geti3()方法做了如下几步：
1. 线程请求获得监视this对象的对象锁（假设未被锁，否则线程等待直到锁释放）
2. 线程内存的数据被消除，从“主”内存区域中读入（Java虚拟机能优化此步。。。[后面的不知道怎么表达,汗]）
3. 代码块被执行
4. 对于变量的任何改变现在可以安全地写到“主”内存区域中（不过geti3()方法不会改变变量值）
5. 线程释放监视this对象的对象锁
　　因此volatile只是在线程内存和“主”内存间同步某个变量的值，而synchronized通过锁定和解锁某个监视器同步所有变量的值。显然synchronized要比volatile消耗更多资源。

附英文原文：
What does volatile do?

This is probably best explained by comparing the effects that volatile and synchronized have on a method. volatile is a field modifier, while synchronized modifies code blocks and methods. So we can specify three variations of a simple accessor using those two keywords:

int i1;              int geti1() {return i1;}
volatile int i2;  int geti2() {return i2;}
int i3;              synchronized int geti3() {return i3;}
geti1() accesses the value currently stored in i1 in the current thread. Threads can have local copies of variables, and the data does not have to be the same as the data held in other threads. In particular, another thread may have updated i1 in it’s thread, but the value in the current thread could be different from that updated value. In fact Java has the idea of a “main” memory, and this is the memory that holds the current “correct” value for variables. Threads can have their own copy of data for variables, and the thread copy can be different from the “main” memory. So in fact, it is possible for the “main” memory to have a value of 1 for i1, for thread1 to have a value of 2 for i1 and for thread2 to have a value of 3 for i1 if thread1 and thread2 have both updated i1 but those updated value has not yet been propagated to “main” memory or other threads.

On the other hand, geti2() effectively accesses the value of i2 from “main” memory. A volatile variable is not allowed to have a local copy of a variable that is different from the value currently held in “main” memory. Effectively, a variable declared volatile must have it’s data synchronized across all threads, so that whenever you access or update the variable in any thread, all other threads immediately see the same value. Of course, it is likely that volatile variables have a higher access and update overhead than “plain” variables, since the reason threads can have their own copy of data is for better efficiency.

Well if volatile already synchronizes data across threads, what is synchronized for? Well there are two differences. Firstly synchronized obtains and releases locks on monitors which can force only one thread at a time to execute a code block, if both threads use the same monitor (effectively the same object lock). That’s the fairly well known aspect to synchronized. But synchronized also synchronizes memory. In fact synchronized synchronizes the whole of thread memory with “main” memory. So executing geti3() does the following:

1. The thread acquires the lock on the monitor for object this (assuming the monitor is unlocked, otherwise the thread waits until the monitor is unlocked).
2. The thread memory flushes all its variables, i.e. it has all of its variables effectively read from “main” memory (JVMs can use dirty sets to optimize this so that only “dirty” variables are flushed, but conceptually this is the same. See section 17.9 of the Java language specification).
3. The code block is executed (in this case setting the return value to the current value of i3, which may have just been reset from “main” memory).
4. (Any changes to variables would normally now be written out to “main” memory, but for geti3() we have no changes.)
5. The thread releases the lock on the monitor for object this.

So where volatile only synchronizes the value of one variable between thread memory and “main” memory, synchronized synchronizes the value of all variables between thread memory and “main” memory, and locks and releases a monitor to boot. Clearly synchronized is likely to have more overhead than volatile.