Java：属性总是返回 null答案

【问题标题】：Java: Properties return always nullJava：属性总是返回 null
【发布时间】：2017-12-29 06:26:30
【问题描述】：

我有问题。 prop.getProperty(key) 始终返回 null。如果我更改属性文件中的某些内容并调用该方法（启动 java 程序），它会重置这些值。我的代码：

    public static boolean checkProperties(){

    Properties prop = new Properties();
    try {

        Writer writer = new FileWriter(propertyFile);
        Reader reader = new FileReader(propertyFile);
        prop.load(reader);


        System.out.println(prop.getProperty("server-port")); // OUTPUT: null; But it is in the file!


        if(prop.getProperty("server-port") == null || prop.getProperty("server-port").equals("")){
            prop.setProperty("server-port", "2424");
            System.out.println("No port was specified in the properties file! Set port to 2424.");
        }
        if(prop.getProperty("info-text") == null || prop.getProperty("info-text").equals("")){
            prop.setProperty("info-text", "Welcome to my Game Server!");
            System.out.println("No info text was specified in the properties file! Set to 'Welcome to my Game Server!'");
        }
        prop.store(writer, "Game Server Config");
        writer.close();
        reader.close();

    } catch (IOException e) {
        System.err.println("Something went wrong while checking properties file!");
        System.err.println(e.getMessage());
        e.printStackTrace();
        return false;
    }

    return true;
}

谢谢， Max2002_。

【问题讨论】：

什么是propertyFile？它指向什么？如果没有这些详细信息，我们无法重现该问题。
File propertyFile = new File("server.properties");
"new FileWriter" 总是创建一个新文件，所以你总是用一个新的空文件覆盖任何现有文件
@gusto2 谢谢你，但我怎样才能改变它，让它工作？
您能告诉我们，您是在哪里创建的属性文件吗？路径是否正确？

标签： java properties null return

【解决方案1】：

试试这个

public static boolean checkProperties() {
        Properties prop = new Properties();
        InputStream input = null;
        boolean flag = false;
        try {
            String filename = "server.properties";
            input = Test.class.getClassLoader().getResourceAsStream(filename);
            prop.load(input);
            System.out.println(prop.getProperty("server-port"));
            if(prop.getProperty("server-port") == null){
                flag=false;
            }
            else{
                flag=true;
            }

        } catch (IOException ex) {
            ex.printStackTrace();
        }
        return flag;
    }

如果您遇到任何问题，请告诉我。

【讨论】：

【解决方案2】：

现在可以了。代码：

    public static boolean checkProperties(){

    Properties prop = new Properties();
    try {

        //Writer writer = new FileWriter(propertyFile);
        FileOutputStream out = null;
        Reader reader = new FileReader(propertyFile);
        prop.load(reader);
        printToConsole(prop.getProperty("server-port"), LEVEL_NORMAL, true);
        if(prop.getProperty("server-port") == null || prop.getProperty("server-port").equals("")){
            prop.setProperty("server-port", "2424");
            printToConsole("No port was specified in the properties file! Set port to 2424.", LEVEL_NORMAL, true);
        }
        if(prop.getProperty("info-text") == null || prop.getProperty("info-text").equals("")){
            prop.setProperty("info-text", "Welcome to my Game Server!");
            printToConsole("No info text was specified in the properties file! Set to 'Welcome to my Game Server!'", LEVEL_NORMAL, true);
        }
        out = new FileOutputStream(propertyFile);
        prop.store(out, "Game Server Config");
        out.close();
        reader.close();

    } catch (IOException e) {
        printToConsole("Something went wrong while checking properties file!", LEVEL_ERROR, true);
        printToConsole(e.getMessage(), LEVEL_ERROR, true);
        return false;
    }

    try{
        port = Integer.parseInt(prop.getProperty("server-port"));
    }catch(Exception e){
        printToConsole("Error: Can not use the port in the properties file ('" + prop.getProperty("server-port") + "')!", LEVEL_ERROR, true);
        printToConsole(e.getMessage(), LEVEL_ERROR, true);
        return false;
    }


    return true;
}

【讨论】：