DDevelopment & Design

Grails render as JSON catch

One of a reasons your controller doesn’t render a proper response in JSON format might be wrong package name that you use. It is easy to overlook. Import are on top of a file, you look at your code and everything seems to be fine. Except response is still not in JSON format. Consider this simple controller: class RestJsonCatchController {def grailsJson() {render([first: 'foo', second: 5] as grails.converters.JSON) }def netSfJson() {render([first: 'foo', second: 5] as net.sf.json.JSON) }} And now, with finger crossed… We have a winner! $ curl localhost:8080/example/restJsonCatch/grailsJson{"first":"foo","second":5}$ curl localhost:8080/example/restJsonCatch/netSfJson{first=foo, second=5} As you can see only grails.converters.JSON converts your response to JSON format. There is no such converter for net.sf.json.JSON, so Grails has no converter to apply and it renders Map normally. Conclusion: always carefully look at your imports if you’re working with JSON in Grails! Edit: Burt suggested that this is a bug. I’ve submitted JIRA issue here: GRAILS-9622 render as class that is not a codec should throw exception

One of a reasons your controller doesn’t render a proper response in JSON format might be wrong package name that you use. It is easy to overlook. Import are on top of a file, you look at your code and everything seems to be fine. Except response is still not in JSON format.

Consider this simple controller:

<span class="keyword">class</span> <span class="class">RestJsonCatchController</span> {<br />    <span class="keyword">def</span> <span class="method">grailsJson</span>() {<br />        <span class="method">render</span>([<span class="field">first</span>: <span class="string">'foo'</span>, <span class="field">second</span>: <span class="number">5</span>] <span class="keyword">as</span> grails.converters.<span class="class">JSON</span>)<br />    }<br /><br />    <span class="keyword">def</span> <span class="method">netSfJson</span>() {<br />        <span class="method">render</span>([<span class="field">first</span>: <span class="string">'foo'</span>, <span class="field">second</span>: <span class="number">5</span>] <span class="keyword">as</span> net.sf.json.<span class="class">JSON</span>)<br />    }<br />}<br />

And now, with finger crossed… We have a winner!

<span class="keyword">$</span> curl localhost:8080/example/restJsonCatch/grailsJson<br />{"first":"foo","second":5}<br /><span class="keyword">$</span> curl localhost:8080/example/restJsonCatch/netSfJson<br />{first=foo, second=5}<br />

As you can see only grails.converters.<span class="class">JSON</span> converts your response to JSON format. There is no such converter for net.sf.json.<span class="class">JSON</span>, so Grails has no converter to apply and it renders Map normally.

Conclusion: always carefully look at your imports if you’re working with JSON in Grails!

Edit: Burt suggested that this is a bug. I’ve submitted JIRA issue here: GRAILS-9622 render as class that is not a codec should throw exception

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DDevelopment & Design

Private fields and methods are not private in groovy

I used to code in Java before I met groovy. Like most of you, groovy attracted me with many enhancements. This was to my surprise to discover that method visibility in groovy is handled different than Java!

Consider this example:

class Person {
private String name
public  String surname

private Person() {}

private String signature() { "${name?.substring(0, 1)}. $surname" }

public String toString() { "I am $name $surname" }
}

How is this class interpreted with Java?

  1. Person has private constructor that cannot be accessed
  2. Field "name" is private and cannot be accessed
  3. Method signature() is private and cannot be accessed

Let's see how groovy interpretes Person:

public static void main(String[] args) {
def person = new Person()   // constructor is private - compilation error in Java
println(person.toString())

    person.@name = 'Mike'       // access name field directly - compilation error in Java
println(person.toString())

    person.name = 'John'        // there is a setter generated by groovy
println(person.toString())

    person.@surname = 'Foo'     // access surname field directly
println(person.toString())

    person.surname = 'Bar'      // access auto-generated setter
println(person.toString())

println(person.signature()) // call private method - compilation error in Java
}

I was really astonished by its output:

I am null null
I am Mike null
I am John null
I am John Foo
I am John Bar
J. Bar

As you can see, groovy does not follow visibility directives at all! It treats them as non-existing. Code compiles and executes fine. It's contrary to Java. In Java this code has several errors, pointed out in comments.

I've searched a bit on this topic and it seems that this behaviour is known since version 1.1 and there is a bug report on that: http://jira.codehaus.org/browse/GROOVY-1875. It is not resolved even with groovy 2 release. As Tim Yates mentioned in this Stackoverflow question: "It's not clear if it is a bug or by design". Groovy treats visibility keywords as a hint for a programmer.

I need to keep that lesson in mind next time I want to make some field or method private!