使用相同的尝试资源读取和写入文件

发布于 2025-02-05 03:17:13 字数 1321 浏览 1 评论 0原文

我正在使用Java，并且有一种方法可以替换configfile中参数的值。我使用try-with-resources，以便两者都会自动关闭。但是，我遇到了出乎意料的行为 - 而循环不会从该文件中读取任何内容，因为在Java进入try-with-with-resources块之后，configfile变为空。

private static boolean replaceValue(String param, String newValue) throws IOException {
    try (BufferedReader br = new BufferedReader(new FileReader(configFile));
         BufferedWriter bw = new BufferedWriter(new FileWriter(configFile))) {
        String line;
        StringBuilder sb = new StringBuilder();
        boolean isParamPresent = false;
        while ((line = br.readLine()) != null) {
            if (line.startsWith(param + configDelimiter)) {
                line = line.replaceAll("(?<==).*", newValue);
                isParamPresent = true;
            }
            sb.append(line);
            sb.append("\n");
        }
        if (isParamPresent) {
            bw.write(sb.toString());
            return true;
        }
    }
    return false;
}

如果我更改为下面的代码，则可以按预期工作。

            if (isParamPresent) {
            try (BufferedWriter bw = new BufferedWriter(new FileWriter(configFile))) {
                bw.write(sb.toString());
                return true;
            }

我不明白什么原因configfile变得空。有人可以解释怎么了吗？

原文

I'm using Java, and I have a method that replaces a value of a parameter in configFile. I use try-with-resources so that both will be closed automatically. However, I encounter an unexpected behavior - the while loop doesn't read anything from that file because immediately after Java enters the try-with-resources block, configFile becomes empty.

private static boolean replaceValue(String param, String newValue) throws IOException {
    try (BufferedReader br = new BufferedReader(new FileReader(configFile));
         BufferedWriter bw = new BufferedWriter(new FileWriter(configFile))) {
        String line;
        StringBuilder sb = new StringBuilder();
        boolean isParamPresent = false;
        while ((line = br.readLine()) != null) {
            if (line.startsWith(param + configDelimiter)) {
                line = line.replaceAll("(?<==).*", newValue);
                isParamPresent = true;
            }
            sb.append(line);
            sb.append("\n");
        }
        if (isParamPresent) {
            bw.write(sb.toString());
            return true;
        }
    }
    return false;
}

If I change to code to be like this below, it works as expected.

            if (isParamPresent) {
            try (BufferedWriter bw = new BufferedWriter(new FileWriter(configFile))) {
                bw.write(sb.toString());
                return true;
            }

I don't understand what causes configFile to become empty. Can someone explain what's wrong?

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那支青花 2025-02-12 03:17:14

filewriter（String fileName） constructor调用fileOutputstream（字符串名称）将Append flag设置为false。这意味着该文件将不会在附录模式下打开。
根据我在Windows上的测试，如果未设置附录标志，则立即清除文件。因此，在您的第一个变体中，自清除以来没有什么可阅读的。您的第二个变体在将内容读取到stringBuffer中时已清除。