Demo

private static String zookeeperHost = System.getProperty("zookeeper.address", "127.0.0.1");
private static String zookeeperPort = System.getProperty("zookeeper.port", "2181");

使用System.getProperty获得属性，如果未获得指定key的值，使用后面的作为default value。

System.getProperty方法支持以下属性：

• java.version：Java Runtime Environment version, which may be interpreted as a Runtime.Version
• java.version.date：Java Runtime Environment version date, in ISO-8601 YYYY-MM-DD format, which may be interpreted as a java.time.LocalDate
• java.vendor：Java Runtime Environment vendor
• java.vendor.url：Java vendor URL
• java.vendor.version：Java vendor version
• java.home：Java installation directory
• java.vm.specification.version：Java Virtual Machine specification version, whose value is the feature element of the runtime version
• java.vm.specification.vendor：Java Virtual Machine specification vendor
• java.vm.specification.name：Java Virtual Machine specification name
• java.vm.version：Java Virtual Machine implementation version which may be interpreted as a Runtime.Version
• java.vm.vendor：Java Virtual Machine implementation vendor
• java.vm.name：Java Virtual Machine implementation name
• java.specification.version：Java Runtime Environment specification version, whose value is the feature element of the runtime version
• java.specification.vendor：Java Runtime Environment specification vendor
• java.specification.name：Java Runtime Environment specification name
• java.class.version：Java class format version number
• java.class.path：Java class path (refer to ClassLoader.getSystemClassLoader() for details)
• java.library.path：List of paths to search when loading libraries
• java.io.tmpdir：Default temp file path
• java.compiler：Name of JIT compiler to use
• os.name：Operating system name
• os.arch：Operating system architecture
• os.version：Operating system version
• file.separator：File separator ("/" on UNIX)
• path.separator：Path separator (":" on UNIX)
• line.separator：Line separator ("\n" on UNIX)
• user.name：User's account name
• user.home：User's home directory
• user.dir：User's current working directory

    public static String getProperty(String key) {
        checkKey(key);
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPropertyAccess(key);
        }

        return props.getProperty(key);
    }

 checkKey(String key)

private static void checkKey(String key) {
        if (key == null) {
            throw new NullPointerException("key can't be null");
        }
        if (key.equals("")) {
            throw new IllegalArgumentException("key can't be empty");
        }
    }

props.getProperty(key)

    public String getProperty(String key) {
        Object oval = map.get(key);
        String sval = (oval instanceof String) ? (String)oval : null;
        Properties defaults;
        return ((sval == null) && ((defaults = this.defaults) != null)) ? defaults.getProperty(key) : sval;
    }