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IO不close的问题

 3 years ago
source link: https://jimolonely.github.io/2019/10/09/java/074-io-close/
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IO不close的问题

文件、网络的输入输出流如果没关闭，会有什么后果？

本文以文件写入为例，实践解释：

BufferedWriter writer = new BufferedWriter(new FileWriter(
        "D:\\data\\a.txt"));
StringBuilder sb = new StringBuilder();
for (int i=0;i<10000;i++>) {
    sb.setLength(0);

for (int j=0;j<100;j++>) {
        sb.append(j+" ");
    }
    String str = sb.toString();
    writer.write(str);
    writer.newLine();
}

以上写入10000行数据，但是结果却没有10000行，总是差一些，为啥？

如果最后加上一行：

writer.close();

那么结果就正确了。

很简单，所谓bufferedWriter，就是缓冲写，也就是会缓存到一定数量才写入文件，最后那一部分数据没写入文件，看close的源码就知道了：

public void close() throws IOException {
    synchronized (lock) {
        if (out == null) {
            return;
        }
        try (Writer w = out) {
            flushBuffer();
        } finally {
            out = null;
            cb = null;
        }
    }
}

void flushBuffer() throws IOException {
    synchronized (lock) {
        ensureOpen();
        if (nextChar == 0)
            return;
        out.write(cb, 0, nextChar);
        nextChar = 0;
    }
}

重点就是flushBuffer方法，最后还有一步写。

因此，一定记得关闭输入输出流。

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