import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

public class CopyImageDemo {

    public static void main(String[] args) {
        FileInputStream fis = null;
        FileOutputStream fos = null;
        try {
            fis = new FileInputStream("c:/a.jpg");
            fos = new FileOutputStream("c:/b.jpg");
            try {
                System.out.println("文件大小：" + fis.available() + "Byte");
                byte buf[] = new byte[fis.available()];
                int len = 0;
                while ((len = fis.read(buf)) != -1) {
                    fos.write(len);
                    fos.flush();
                    
                }

            } catch (IOException e) {

                e.printStackTrace();
            }
        } catch (FileNotFoundException e) {

            e.printStackTrace();
        } finally {
            try {
                if (fis != null) {
                    fis.close();
                }
                if (fos != null) {
                    fos.close();
                }
            } catch (IOException e) {

                e.printStackTrace();
            }
        }
    }
}

            // 新建文件输入流并对它进行缓冲
            inBuff = new BufferedInputStream(new FileInputStream(sourceFile));

            // 新建文件输出流并对它进行缓冲
            outBuff = new BufferedOutputStream(new FileOutputStream(targetFile));

            // 缓冲数组
            byte[] b = new byte[1024 * 5];
            int len;
            while ((len = inBuff.read(b)) != -1) {
                outBuff.write(b, 0, len);
            }
            // 刷新此缓冲的输出流
            outBuff.flush();

------解决方案--------------------
建议：
用BufferedInputStream包装FileInputStream
用BufferedOutputStream包装FileOutputStream
flush方法写在while循环的外面