For UNIX systems, shared memory and shared memory is divided into general image files shared memory are two types of windows is actually only one kind of shared memory image file. So java applications also can create image files shared memory. Use shared memory, it has the following characteristics:
1, can be opened to access multiple processes.
2, reading and writing processes when performing read and write operations other processes can not write.
3, multiple processes can write to a shared memory alternately executed right.
4, after a process performed memory write operation, it does not affect access to the memory of other processes. While other processes have visibility memory after the update.
5, when the process executes a write operation, if abnormal exit, the write operation to other processes ban should automatically removed.
Generally, we operate shared memory in the following situations, the main concern 1,2:
1, an exclusive write operation, there is a corresponding exclusive write queue. Exclusive write consistency of the data in question will not happen itself.
2, the shared write operation, the write operation corresponding with a shared queue. Share writes Oh will have to be taken to prevent data consistency problems.
3, exclusive read operation, the read operation corresponding with a shared queue.
4, shared read operation, the read operation corresponding with a shared queue.
Class MappedByteBuffer provided in jdk1.4 provides us with a way to achieve shared memory, the memory buffer is actually a disk image file. Changes both to keep pace, that is, memory data changes will immediately react to a disk file, it will effectively ensure the realization of shared memory.
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;
import java.util.Properties;
/**
* 共享内存操作类
* @author hx
*
*/
public class ShareMemory {
int flen = 41779264; //开辟共享内存大小
int fsize = 0; //文件的实际大小
String shareFileName; //共享内存文件名
String sharePath; //共享内存路径
MappedByteBuffer mapBuf = null; //定义共享内存缓冲区
FileChannel fc = null; //定义相应的文件通道
FileLock fl = null; //定义文件区域锁定的标记。
Properties p = null;
RandomAccessFile RAFile = null; //定义一个随机存取文件对象
/**
*
* @param sp 共享内存文件路径
* @param sf 共享内存文件名
*/
public ShareMemory(String sp, String sf) {
if (sp.length() != 0) {
File folder = new File(sp);
if(!folder.exists()){
folder.mkdirs();
}
}
this.sharePath = sp + File.separator;
this.shareFileName = sf;
try {
// 获得一个只读的随机存取文件对象 "rw" 打开以便读取和写入。如果该文件尚不存在，则尝试创建该文件。
RAFile = new RandomAccessFile(this.sharePath + this.shareFileName + ".sm", "rw");
//获取相应的文件通道
fc = RAFile.getChannel();
//获取实际文件的大小
fsize = (int) fc.size();
if (fsize < flen) {
byte bb[] = new byte[flen - fsize];
//创建字节缓冲区
ByteBuffer bf = ByteBuffer.wrap(bb);
bf.clear();
//设置此通道的文件位置。
fc.position(fsize);
//将字节序列从给定的缓冲区写入此通道。
fc.write(bf);
fc.force(false);
fsize = flen;
}
//将此通道的文件区域直接映射到内存中。
mapBuf = fc.map(FileChannel.MapMode.READ_WRITE, 0, fsize);
} catch (IOException e) {
e.printStackTrace();
}
}
/**
*
* @param ps 锁定区域开始的位置；必须为非负数
* @param len 锁定区域的大小；必须为非负数
* @param buff 写入的数据
* @return
*/
public synchronized int write(int ps, int len, byte[] buff) {
if (ps >= fsize || ps + len >= fsize) {
return 0;
}
//定义文件区域锁定的标记。
FileLock fl = null;
try {
//获取此通道的文件给定区域上的锁定。
fl = fc.lock(ps, len, false);
if (fl != null) {
mapBuf.position(ps);
ByteBuffer bf1 = ByteBuffer.wrap(buff);
mapBuf.put(bf1);
//释放此锁定。
fl.release();
return len;
}
} catch (Exception e) {
if (fl != null) {
try {
fl.release();
} catch (IOException e1) {
System.out.println(e1.toString());
}
}
return 0;
}
return 0;
}
/**
*
* @param ps 锁定区域开始的位置；必须为非负数
* @param len 锁定区域的大小；必须为非负数
* @param buff 要取的数据
* @return
*/
public synchronized int read(int ps, int len, byte[] buff) {
if (ps >= fsize) {
return 0;
}
//定义文件区域锁定的标记。
FileLock fl = null;
try {
fl = fc.lock(ps, len, false);
if (fl != null) {
//System.out.println( "ps="+ps );
mapBuf.position(ps);
if (mapBuf.remaining() < len) {
len = mapBuf.remaining();
}
if (len > 0) {
mapBuf.get(buff, 0, len);
}
fl.release();
return len;
}
} catch (Exception e) {
if (fl != null) {
try {
fl.release();
} catch (IOException e1) {
System.out.println(e1.toString());
}
}
return 0;
}
return 0;
}
/**
* 关闭共享内存操作
*/
public synchronized void closeSMFile() {
if (fc != null) {
try {
fc.close();
} catch (IOException e) {
System.out.println(e.toString());
}
fc = null;
}
if (RAFile != null) {
try {
RAFile.close();
} catch (IOException e) {
System.out.println(e.toString());
}
RAFile = null;
}
mapBuf = null;
}
/**
* 检查退出
* @return true-成功，false-失败
*/
public synchronized boolean checkToExit() {
byte bb[] = new byte[1];
if (read(1, 1, bb) > 0) {
if (bb[0] == 1) {
return true;
}
}
return false;
}
/**
* 复位退出
*/
public synchronized void resetExit() {
byte bb[] = new byte[1];
bb[0] = 0;
write(1, 1, bb);
}
/**
* 退出
*/
public synchronized void toExit() {
byte bb[] = new byte[1];
bb[0] = 1;
write(1, 1, bb);
}
public static void main(String arsg[]) throws Exception{
ShareMemory sm = new ShareMemory("E://demo","test");
String str = "中文测试";
byte[] bytes = str.getBytes("UTF-8");
int byteLength = bytes.length;
System.out.println(byteLength);
sm.write(40, byteLength, bytes);
byte[] b = new byte[byteLength];
sm.read(40, byteLength, b);
System.out.println(new String(b,"UTF-8"));
}
}