how to get a real null value instead of a JSONObje

问题:

I'm trying to parse some JSON in Grails using the grails.converters.JSON library. I have a field which will contain either a string, or a null value. When I parse the JSON and get the field, the null values come back as a JSONObject.NULL type. This is not good when checking != null as JSONObject.NULL is evaluated as non-null (not good for null checks)

def obj = JSON.parse('{"date1":null,"date2":"2011-06-26T05:00:00Z"}')
def date1 = obj.date1
if (date1)
     parse(date1)   // parse error occurs here because date1 evaluates true in the if because it is JSONObject.NULL

Is there an easy way to get the parse to parse a real null value so that I don't have to check if the object is a JSONObject.NULL.

I tried the suggestion here to use .toString(), but it ended up returning the string value 'null' instead of actual null for a JSONObject.NULL value.

回答1:

Have a look at: http://grails.1312388.n4.nabble.com/The-groovy-truth-of-JSONObject-Null-td3661040.html

Ian Roberts mentions a nice trick to make a null check possible:

JSONObject.NULL.metaClass.asBoolean = {-> false} 

回答2:

You may find this more useful and natural

    JSONObject.NULL.equals(jsonObj.get("key_name"))

回答3:

I think I found a better solution, which consists in overriding the toString() method implementation of the JSONObject.NULL inner class by copying the JSONObject.java file into your Grails src/java project and then changing the implementation to this:

    /**
     * Get the "" string value.
     * @return An empty String "".
     */
    @Override
    public String toString() {
        return "";
    }

Once you restart with this new class in your classpath, the classloader will use your JSONObject class instead of the one packaged in the Grails dependencies.

Make sure you keep it in the same package as the original.

For more details you can go here: https://github.com/grails/grails-core/issues/9129

Hope it helps :-)