用自定义字符串替换美元符号（$）变量的Java EE方法

Question

Is there a pre-written method to replace dollar-sign name variables in a string with a predefined constant?

For example, the following code :

Map<String, Object> myVars = new TreeMap<String, Object>();
String str = "The current year is ${currentYear}.";
myVars.put("currentYear", "2014");
System.out.println(Replacer.replaceVars(str, myVars));

... would have this output:

The current year is 2014.

Answer 1

Have a look at the MessageFormat class. Your example would look like this:

int currentYear = 2014;
String str = "The current year is {0}.";
str = MessageFormat.format(str, currentYear);

This is probably the best version, but you could always use a regex as well:

public String format (String input, Map<String, String> replacement)
{
    for (String key : replacement.keySet())
        input = input.replaceAll("\\${"+replacement+"}", replacement.get(key));
    return input;
}

Answer 2

Spring does this too if you need to support more advanced use cases. I was able to utilize code from the following class for my use cases. See the parseStringValue method.

https://github.com/spring-projects/spring-framework/blob/master/spring-core/src/main/java/org/springframework/util/PropertyPlaceholderHelper.java

In your case, you need to pass in a PlaceholderResolver that uses your Map to resolve the placeholder.

Answer 3

I am not aware of any standard Java class which would support this behaviour but writing your tool wouldn't be so hard. Here is example of such solution:

class Replacer {
    private static Pattern pattern = Pattern.compile("\\$\\{(?<key>[^}]*)\\}");

    public static String replaceVars(String format, Map<String, ?> map) {
        StringBuffer sb = new StringBuffer();
        Matcher m = pattern.matcher(format);
        while (m.find()) {
            String key = m.group("key");
            if (map.containsKey(key)) {//replace if founded key exists in map
                m.appendReplacement(sb, map.get(key).toString());
            } else {//do not replace, or to be precise replace with same value
                m.appendReplacement(sb, m.group());
            }
        }
        m.appendTail(sb);

        return sb.toString();
    }
}

We could take advantage of default method getOrDefault introduced in Java 8 to Map interface and replace

while (m.find()) {
    String key = m.group("key");
    if (map.containsKey(key)) {//replace if founded key exists in map
        m.appendReplacement(sb, map.get(key).toString());
    } else {//do not replace, or to be precise replace with same value
        m.appendReplacement(sb, m.group());
    }
}
m.appendTail(sb);

with

while (m.find()) 
    m.appendReplacement(sb, map.getOrDefault(m.group("key"), m.group()));
m.appendTail(sb);

But to be able to use this method we first would need to specify type of value in map - in other words we would need to change type of accepted Map from Map<String, ?> map to type like Map<String, String> map .