java 读/写构造

Question

谁能解释一下为什么这个结构不起作用：

while (fileInputStream.available()>0) {    
    fileOutputStream.write(fileInputStream.read());
}

这个效果很好：

while (fileInputStream.available()>0) {
    int data = fileInputStream.read();
    fileOutputStream.write(data);
}

对我来说它们是相同的，但是第一个不能正确写入数据（将写入文件长度/数据的一半）。

Answer 1

我的兴趣被激起了，我写了这个小测试：

public static void main(String[] args) throws IOException {

    FileInputStream fileInputStream = new FileInputStream("/home/nick/foo");
    FileOutputStream fileOutputStream = new FileOutputStream("/home/nick/bar");
    fileOutputStream.write(fileInputStream.read());

    fileOutputStream.flush();
    fileOutputStream.close();
    fileInputStream.close();
}

它按预期工作 - 从 /home/nick/foo 读取一个字节并将其写入 /home/nick/bar

编辑：

更新程序：

public static void main(String[] args) throws IOException {

    FileInputStream fileInputStream = new FileInputStream("/home/nick/foo");
    FileOutputStream fileOutputStream = new FileOutputStream("/home/nick/bar");
    while (fileInputStream.available()>0) {    
        fileOutputStream.write(fileInputStream.read());
    }

    fileOutputStream.flush();
    fileOutputStream.close();
    fileInputStream.close();
}

复制了整个文件。（注意 - 我不建议一次复制一个字节的文件，使用 Buffered I/O 类来复制整个块）

你有没有忘记 flush() 和 close() 到 OutputStream？

Answer 2

看起来你的 while 循环正在做估计的事情并在这里和那里跳过。

public int available() throws IOException {
        return 0;
    }

Returns an estimate of the number of remaining bytes that can be read (or skipped over) from this input stream without blocking by the next invocation of a method for this input stream. The next invocation might be the same thread or another thread. A single read or skip of this many bytes will not block, but may read or skip fewer bytes. In some cases, a non-blocking read (or skip) may appear to be blocked when it is merely slow, for example when reading large files over slow networks.

See here

Answer 3

您使用的 available() 方法不正确。该方法用于确定在不阻塞线程的情况下有多少字节可供读取。

Good Stack Overflow Question about available()

JavaDoc on available()

检查是否到达EOF的正确方法是查看read()方法是否返回-1:

int data = fileInputStream.read();
while (data != -1) {
    fileOutputStream.write(data);
    data = fileInputStream.read();
}

如果您尝试读取大量数据，此方法可能会非常慢。您可以使用 read(byte[] b, int off, int len) 方法读取更多字节来加快速度。该循环看起来与另一个循环非常相似。

byte [] buffer = new byte[1024]; // 1kb buffer
int numBytesRead = fileInputStream.read(buffer);
while (numBytesRead != -1) {
    fileOutputStream.write(buffer, 0, numBytesRead);
    numBytesRead = fileInputStream.read(buffer);
}

java 读/写构造

java read / write construction

java

io

fileinputstream

fileoutputstream