Java 文件与字节流(InputStream.available()在本地文件和网络文件的异同)

1 文件到字节流

1.1 IO

IO：输入流,直接写完close即可

InputStream in = new FileInputStream(fileName);
DataInputStream stream = new DataInputStream(in);

IO：输出流，直接读完close即可,即使FileOutputStream(String name, boolean append)可以有append参数但是这个跟Files相比还是弱

FileOutputStream fileOut = new FileOutputStream(fileName);
DataOutputStream stream = new DataOutputStream(fileOut)

1.2 NIO 借助java.nio.file.Files

NIO：输入流

InputStream in = Files.newInputStream(Paths.get(fileName), StandardOpenOption.READ);

NIO：输出流

OutputStream out =  Files.newOutputStream(Paths.get(fileName));//StandardOpenOption.CREATE,没有参数会无视有没有文件存在都trunte并rewrite

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2 文件和byte[]

2.1 将byte[]写入文件

Files.write(Paths.get(fileName), data, options);

2.2 从文件获取所有的byte[]

Files.readAllBytes(Paths.get(fileName));

2.3 或者参考

public static byte[] nioRead(String file){ 
    byte[] b = null;
    try(FileInputStream in = new FileInputStream(file)) {
        FileChannel channel = in.getChannel();  //FileInputStream.getChannel()
        ByteBuffer buffer = ByteBuffer.allocate(1024);  
        channel.read(buffer);  
        b = buffer.array();  
        return b;
    } catch (Exception e) {
        LogCore.BASE.error("nioRead err:{}", e);
        return b;
    }
} 

3 从InputStream流到byte[] is.available()是全部字节码

请看下面代码

public static byte[] readAllBytes(InputStream is){
    byte[] data = null;
    try {
        if(is.available()==0){//返回null而不是空byte[]
            return data;
        }
        data = new byte[is.available()];
        is.read(data);
        is.close();
        return data;
    } catch (IOException e) {
        e.printStackTrace();
        return data;
    }
}
注意这个方法只能正确读取本地文件的流is.available()就是全部的字节数。
如果是网络过来的流的话is.available()可就不是全部字节数了，而只是当前可读的字节数
正确的例子如下
思路是不断的读取byte[1024]然后组成新数组byte[total]也就是不断用System.arrayCopy去合并数组
其实这也是字节流的过程！！！
因此下发如下
public boolean save(String filename, String contentType, InputStream is) {
        try {
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            byte[] buffer = new byte[1024];
            int len = 0;
            while ((len = is.read(buffer)) != -1) {
                baos.write(buffer, 0, len);
            }
            byte[] data = baos.toByteArray();
            Files.write(Paths.get(rootPath, filename), data);
            is.close();
            baos.close();
            return true;
        } catch (IOException e) {
            LogCore.BASE.error("save file err", e);
            return false;
        }
    }

4 字节流和byte[]

• 可以根据这个自定义序列化方案

4.1 字节流写入并提取byte[]

ByteArrayOutputStream bout = new ByteArrayOutputStream();
DataOutputStream stream = new DataOutputStream(bout);
stream.writeInt(v);
...
bout.toByteArray();

4.2 读byte[]数据到字节流

ByteArrayInputStream in = new ByteArrayInputStream(data);
DataInputStream stream = new DataInputStream(in);
int wireFormat = stream.readInt();
...

5 移动文件

Files.move(Paths.get(srcName), Paths.get(tarName));//如果存在java.nio.file.FileAlreadyExistsException
Files.move(Paths.get(srcName), Paths.get(newname), StandardCopyOption.REPLACE_EXISTING);//如果目标文件存在则被替换

5 其他

5.1 判断文件是否存在

Files.exists(Paths.get(fileName));