如果要实现多线程下载，可以定义以下步骤：

1）创建URL对象（URL代表统一资源定位器）

URL url=new URL(path);//path资源路径

2）获取URL对象指向资源的大小，使用URLConnection类，该类代表应用程序和URL之间的通信链接。

                HttpURLConnection conn=(HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");//以GET方式获取
conn.setRequestProperty("Accept-Language", "zh-CN");
conn.setRequestProperty("Charset", "UTF-8");
conn.setRequestProperty("Connection", "Keep-Alive");
fileSize=conn.getContentLength();//得到文件大小

conn.disconnect();

3）在本地磁盘上创建一个与网络资源相同大小的空文件

               RandomAccessFile files=new RandomAccessFile(targetFile, "rw");
files.setLength(fileSize);//设置文件大小
files.close();//关闭文件      

4）创建线程，计算每个线程下载资源的大小并启动线程

代码如下：

import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

public class DownUtil {
	private String path;//下载地址
	private String targetFile;//文件保存位置
	private int threadNumber;//下载的线程数
	private int fileSize;//文件总大小
	private DownThread[] downThreads;//下载线程对象
	
	public DownUtil(String path,String targetFile,int threadNumber){
		this.path=path;
		this.targetFile=targetFile;
		this.threadNumber=threadNumber;
		downThreads=new DownThread[threadNumber];//初始化数组
	}
	public void download() throws Exception{
		URL url=new URL(path);
		HttpURLConnection conn=(HttpURLConnection) url.openConnection();
		conn.setRequestMethod("GET");
		conn.setRequestProperty("Accept-Language", "zh-CN");
		conn.setRequestProperty("Charset", "UTF-8");
		conn.setRequestProperty("Connection", "Keep-Alive");
		fileSize=conn.getContentLength();//得到文件大小
		conn.disconnect();
		int currentPartSize=fileSize/threadNumber+1;//每线程对象下载文件的大小
		RandomAccessFile files=new RandomAccessFile(targetFile, "rw");
		files.setLength(fileSize);//设置文件大小
		files.close();//关闭文件
		for(int i=0;i<threadNumber;i++){
			int start=i+currentPartSize;//每个线程下载的起始位置
			RandomAccessFile file=new RandomAccessFile(targetFile, "rw");
			file.seek(start);//定位下载位置
			
			downThreads[i]=new DownThread(path,start,currentPartSize,file);
			downThreads[i].start();//启动线程
		}
		
	}
	
	
}

import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

public class DownThread extends Thread{
	private int start;//下载线程起始位置
	private int currentPartSize;//该线程下载文件的大小
	private RandomAccessFile file;//存放的文件
	private String path;//下载地址
	private int length;//该线程已经下载的文件大小
	
	public DownThread(String path,int start,int currentPartSize,RandomAccessFile file){
		this.start=start;
		this.currentPartSize=currentPartSize;
		this.file=file;
		this.path=path;
	}
	@Override
	public void run() {
		URL url;
		try {
			url = new URL(path);
			HttpURLConnection conn=(HttpURLConnection) url.openConnection();
			conn.setRequestMethod("GET");
			conn.setRequestProperty("Accept-Language", "zh-CN");
			conn.setRequestProperty("Charset", "UTF-8");
			conn.setRequestProperty("Connection", "Keep-Alive");
			InputStream in=conn.getInputStream();
			in.skip(start);//跳过start个字节，每个线程只负责下载自己的文件
			byte[] buffer=new byte[1024];
			int hasRead=0;
			while(length<currentPartSize&&(hasRead=in.read(buffer))!=-1){
				file.write(buffer, 0, hasRead);
				length+=hasRead;
			}
			file.close();
			in.close();
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
		
	}
}

转载请注明出处：http://blog.csdn.net/hai_qing_xu_kong/article/details/40783867     情绪控_

Java网络编程笔记2