Properties prop = new Properties();
String fileName = "./src/test/resources/predefined.properties";
FileInputStream inputStream = new FileInputStream(fileName);
InputStreamReader reader = new InputStreamReader(inputStream,"UTF-8");

不管怎样，我的问题是文件本身的编码是错误的。使用iconv对我很有效

iconv -f ISO-8859-15 -t UTF-8  messages_nl.properties > messages_nl.properties.new

package com.varaneckas.utils;  

import java.io.UnsupportedEncodingException;  
import java.util.Enumeration;  
import java.util.PropertyResourceBundle;  
import java.util.ResourceBundle;  

/** 
 * UTF-8 friendly ResourceBundle support 
 *  
 * Utility that allows having multi-byte characters inside java .property files. 
 * It removes the need for Sun's native2ascii application, you can simply have 
 * UTF-8 encoded editable .property files. 
 *  
 * Use:  
 * ResourceBundle bundle = Utf8ResourceBundle.getBundle("bundle_name"); 
 *  
 * @author Tomas Varaneckas <tomas.varaneckas@gmail.com> 
 */  
public abstract class Utf8ResourceBundle {  

    /** 
     * Gets the unicode friendly resource bundle 
     *  
     * @param baseName 
     * @see ResourceBundle#getBundle(String) 
     * @return Unicode friendly resource bundle 
     */  
    public static final ResourceBundle getBundle(final String baseName) {  
        return createUtf8PropertyResourceBundle(  
                ResourceBundle.getBundle(baseName));  
    }  

    /** 
     * Creates unicode friendly {@link PropertyResourceBundle} if possible. 
     *  
     * @param bundle  
     * @return Unicode friendly property resource bundle 
     */  
    private static ResourceBundle createUtf8PropertyResourceBundle(  
            final ResourceBundle bundle) {  
        if (!(bundle instanceof PropertyResourceBundle)) {  
            return bundle;  
        }  
        return new Utf8PropertyResourceBundle((PropertyResourceBundle) bundle);  
    }  

    /** 
     * Resource Bundle that does the hard work 
     */  
    private static class Utf8PropertyResourceBundle extends ResourceBundle {  

        /** 
         * Bundle with unicode data 
         */  
        private final PropertyResourceBundle bundle;  

        /** 
         * Initializing constructor 
         *  
         * @param bundle 
         */  
        private Utf8PropertyResourceBundle(final PropertyResourceBundle bundle) {  
            this.bundle = bundle;  
        }  

        @Override  
        @SuppressWarnings("unchecked")  
        public Enumeration getKeys() {  
            return bundle.getKeys();  
        }  

        @Override  
        protected Object handleGetObject(final String key) {  
            final String value = bundle.getString(key);  
            if (value == null)  
                return null;  
            try {  
                return new String(value.getBytes("ISO-8859-1"), "UTF-8");  
            } catch (final UnsupportedEncodingException e) {  
                throw new RuntimeException("Encoding not supported", e);  
            }  
        }  
    }  
}  

我尝试使用Rod提供的方法，但考虑到BalusC关注的是不要在所有应用程序中重复相同的工作，并附带了这个类:

import java.io.UnsupportedEncodingException;
import java.util.Locale;
import java.util.ResourceBundle;

public class MyResourceBundle {

    // feature variables
    private ResourceBundle bundle;
    private String fileEncoding;

    public MyResourceBundle(Locale locale, String fileEncoding){
        this.bundle = ResourceBundle.getBundle("com.app.Bundle", locale);
        this.fileEncoding = fileEncoding;
    }

    public MyResourceBundle(Locale locale){
        this(locale, "UTF-8");
    }

    public String getString(String key){
        String value = bundle.getString(key); 
        try {
            return new String(value.getBytes("ISO-8859-1"), fileEncoding);
        } catch (UnsupportedEncodingException e) {
            return value;
        }
    }
}

使用它的方式与常规的ResourceBundle使用非常相似:

private MyResourceBundle labels = new MyResourceBundle("es", "UTF-8");
String label = labels.getString(key)

或者你可以使用默认使用UTF-8的替代构造函数:

private MyResourceBundle labels = new MyResourceBundle("es");

Java 9及更新版本

从Java 9开始，属性文件默认编码为UTF-8，使用ISO-8859-1以外的字符应该可以开箱即用。

Java 8及以上版本

当指定了.properties文件时，ResourceBundle#getBundle()在封面下使用PropertyResourceBundle。这反过来使用默认的Properties#load(InputStream)来加载这些属性文件。根据javadoc，它们默认读取为ISO-8859-1。

public void load(InputStream stream)抛出IOException

从输入字节流中读取属性列表(键和元素对)。输入流采用load(Reader)中指定的简单的面向行的格式，并假定使用ISO 8859-1字符编码;即每个字节是一个拉丁字符。非拉丁字符1和某些特殊字符使用Java™语言规范3.3节中定义的Unicode转义符在键和元素中表示。

So, you'd need to save them as ISO-8859-1. If you have any characters beyond ISO-8859-1 range and you can't use \uXXXX off top of head and you're thus forced to save the file as UTF-8, then you'd need to use the native2ascii tool to convert an UTF-8 saved properties file to an ISO-8859-1 saved properties file wherein all uncovered characters are converted into \uXXXX format. The below example converts a UTF-8 encoded properties file text_utf8.properties to a valid ISO-8859-1 encoded properties file text.properties.

native2ascii -encoding UTF-8 text_utf8.properties text.properties

在使用Eclipse等正常的IDE时，当您在基于Java的项目中创建.properties文件并使用Eclipse自己的编辑器时，这已经自动完成了。Eclipse将透明地将超出ISO-8859-1范围的字符转换为\uXXXX格式。另见下面的截图(注意底部的“属性”和“源”选项卡，点击放大):

或者，您也可以创建一个定制的ResourceBundle。控件实现，其中您显式读取属性文件为UTF-8使用InputStreamReader，这样您就可以将它们保存为UTF-8而不需要与native2ascii的麻烦。下面是一个开始的例子:

public class UTF8Control extends Control {
    public ResourceBundle newBundle
        (String baseName, Locale locale, String format, ClassLoader loader, boolean reload)
            throws IllegalAccessException, InstantiationException, IOException
    {
        // The below is a copy of the default implementation.
        String bundleName = toBundleName(baseName, locale);
        String resourceName = toResourceName(bundleName, "properties");
        ResourceBundle bundle = null;
        InputStream stream = null;
        if (reload) {
            URL url = loader.getResource(resourceName);
            if (url != null) {
                URLConnection connection = url.openConnection();
                if (connection != null) {
                    connection.setUseCaches(false);
                    stream = connection.getInputStream();
                }
            }
        } else {
            stream = loader.getResourceAsStream(resourceName);
        }
        if (stream != null) {
            try {
                // Only this line is changed to make it to read properties files as UTF-8.
                bundle = new PropertyResourceBundle(new InputStreamReader(stream, "UTF-8"));
            } finally {
                stream.close();
            }
        }
        return bundle;
    }
}

可以这样使用:

ResourceBundle bundle = ResourceBundle.getBundle("com.example.i18n.text", new UTF8Control());

参见:

Unicode -如何得到正确的字符?

注意:在Java <= 8 Java属性文件应该编码在ISO 8859-1!

ISO 8859-1字符编码。 不能直接输入的字符 用这种编码表示可以 使用Unicode转义符编写;只有 允许输入一个“u”字符 转义序列。

Java文档

如果你真的想这样做:看看: Eclipse中的Java属性UTF-8编码——有一些代码示例

因为Java 9:属性文件是用UTF-8编码的，所以应该没有问题/疑问

在Java SE 9中，属性文件以UTF-8编码方式加载。在以前的版本中，ISO-8859-1编码用于加载属性资源包。

(https://docs.oracle.com/javase/9/intl/internationalization -改进- jdk 9. - htm # jsint guid - 9 - dcdb41c a989 - 4220 - 8140 - dbfb844a0fca)