Question
How can I overwrite a method in Groovy if the class implements an interface? If the class does not implement an interface I can overwrite the method, but if the class implements an interface the method does not overwrite.
Example without implement
interface IA {
void abc()
}
class A {
void abc() {
println "original"
}
}
x= new A()
x.abc()
x.metaClass.abc = {-> println "new" }
x.abc()
The output of this is
original
new
Example with implement
Consider the following example where class A implements interface IA
interface IA {
void abc()
}
class A implements IA {
void abc() {
println "original"
}
}
x= new A()
x.abc()
x.metaClass.abc = {-> println "new" }
x.abc()
The output in this case is
original
original
As mentioned in the comments, looks like this is a pretty embarrassing bug, unfortunately.
But depending on your actual scenario, it may be possible to work around the problem.
Use Groovy #Category
If you can control where the method will be called, then you can use a Groovy Category to replace the method within a block:
x= new A()
x.abc()
#Category(A)
class ReplaceAbc {
void abc() {
println 'new'
}
}
use(ReplaceAbc) {
x.abc()
}
Replace the instance with an anonymous sub-type of the original type
If you can re-assign the variable, then this is an obvious way to override the method:
x= new A()
x.abc()
x = new A() {
#Override
void abc() {
println 'new'
}
}
x.abc()
How can I overwrite a method in Groovy if the class implements an
interface?
There are a few ways to do it. One is with runtime extension methods.
See the project at github.com/jeffbrown/victorchoymetaprogramminginterface.
lib/src/main/groovy/victorchoymetaprogramminginterface/IA.groovy
package victorchoymetaprogramminginterface
interface IA {
String abc()
}
lib/src/main/groovy/victorchoymetaprogramminginterface/A.groovy
package victorchoymetaprogramminginterface
class A implements IA{
#Override
String abc() {
'A.abc'
}
}
lib/src/main/groovy/victorchoymetaprogramminginterface/extensions/SomeInterfaceExtensionMethods.groovy
package victorchoymetaprogramminginterface.extensions
import victorchoymetaprogramminginterface.IA
class SomeInterfaceExtensionMethods {
static String abc(IA ia) {
'SomeInterfaceExtensionMethods.abc'
}
}
lib/src/main/resources/META-INF/services/org.codehaus.groovy.runtime.ExtensionModule
moduleVersion=1.0
moduleName=Demo Extensions
staticExtensionClasses=victorchoymetaprogramminginterface.extensions.SomeInterfaceExtensionMethods
The following test passes:
lib/src/test/groovy/victorchoymetaprogramminginterface/TestInterfaceExtensions.groovy
package victorchoymetaprogramminginterface
import spock.lang.Specification
class TestInterfaceExtensions extends Specification {
def "test interface extensions"() {
setup:
def obj = new A()
expect:
obj.abc() == 'SomeInterfaceExtensionMethods.abc'
}
}
Related
I have the following code:
import groovy.transform.ToString
#ToString(includeNames = true)
class Simple {
String creditPoints
}
Simple simple = new Simple()
simple.with {
creditPoints : "288"
}
println simple
Clearly, I made a mistake here with creditPoints : "288". It should have been creditPoints = "288".
I expected Groovy to fail at the runtime saying that I made a mistake and I should have used creditPoints = "288"but clearly it did not.
Since it did not fail then what did Groovy do with the closure I created?
From the Groovy compiler perspective, there is no mistake in your closure code. The compiler sees creditPoints : "288" as labeled statement which is a legal construction in the Groovy programming language. As the documentation says, label statement does not add anything to the resulting bytecode, but it can be used for instance by AST transformations (Spock Framework uses it heavily).
It becomes more clear and easy to understand if you format code more accurately to the label statement use case, e.g
class Simple {
String creditPoints
static void main(String[] args) {
Simple simple = new Simple()
simple.with {
creditPoints:
"288"
}
println simple
}
}
(NOTE: I put your script inside the main method body to show you its bytecode representation in the next section.)
Now when we know how compiler sees this construction, let's take a look and see what does the final bytecode look like. To do this we will decompile the .class file (I use IntelliJ IDEA for that - you simply open .class file in IDEA and it decompiles it for you):
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
import groovy.lang.Closure;
import groovy.lang.GroovyObject;
import groovy.lang.MetaClass;
import groovy.transform.ToString;
import org.codehaus.groovy.runtime.DefaultGroovyMethods;
import org.codehaus.groovy.runtime.GeneratedClosure;
import org.codehaus.groovy.runtime.InvokerHelper;
#ToString
public class Simple implements GroovyObject {
private String creditPoints;
public Simple() {
MetaClass var1 = this.$getStaticMetaClass();
this.metaClass = var1;
}
public static void main(String... args) {
Simple simple = new Simple();
class _main_closure1 extends Closure implements GeneratedClosure {
public _main_closure1(Object _outerInstance, Object _thisObject) {
super(_outerInstance, _thisObject);
}
public Object doCall(Object it) {
return "288";
}
public Object call(Object args) {
return this.doCall(args);
}
public Object call() {
return this.doCall((Object)null);
}
public Object doCall() {
return this.doCall((Object)null);
}
}
DefaultGroovyMethods.with(simple, new _main_closure1(Simple.class, Simple.class));
DefaultGroovyMethods.println(Simple.class, simple);
Object var10000 = null;
}
public String toString() {
StringBuilder _result = new StringBuilder();
Boolean $toStringFirst = Boolean.TRUE;
_result.append("Simple(");
if ($toStringFirst == null ? false : $toStringFirst) {
Boolean var3 = Boolean.FALSE;
} else {
_result.append(", ");
}
if (this.getCreditPoints() == this) {
_result.append("(this)");
} else {
_result.append(InvokerHelper.toString(this.getCreditPoints()));
}
_result.append(")");
return _result.toString();
}
public String getCreditPoints() {
return this.creditPoints;
}
public void setCreditPoints(String var1) {
this.creditPoints = var1;
}
}
As you can see, your closure used with the with method is represented as an inner _main_closure1 class. This class extends Closure class, and it implements GeneratedClosure interface. The body of the closure is encapsulated in public Object doCall(Object it) method. This method only returns "288" string, which is expected - the last statement of the closure becomes a return statement by default. There is no label statement in the generated bytecode, which is also expected as labels get stripped at the CANONICALIZATION Groovy compiler phase.
I want to attach public method to the class.
This is called extension method in C#.
package extensionMethods
class A {
def testA() {}
//def testB() {} Need to add a public method to this class A but we don't have access to the class
}
class B {
def test() {
def a = new A();
a.testA()
a.testB() //Need to add a public method to the Class A without defining the method in the class A
}
}
//In C# way -> Extension method
class C {
/* void testB(this A a) {
}*/
}
How can we achieve the similar approach in Groovy?
In the above example I want to attach method testB() to class A
You will want something like this:
package something
class SomeExtensionClass {
static void testB(A self) {
// ...
}
}
Then your META-INF/services/org.codehaus.groovy.runtime.ExtensionModule extension descriptor...
moduleName=Test module for specifications
moduleVersion=1.0-test
extensionClasses=something.SomeExtensionClass
See http://groovy-lang.org/metaprogramming.html#_extension_modules for more info.
I'm going to internationalize groovy API abit.
For final class (e.g. String)
String.metaClass.вСтроку = {-> this.toString() }
However, this will create additional closure. Isn't there any way to just alias method with another method?
Something like this:
String.metaClass.вСтроку = String.metaClass.&toString
You could use #Category transform like this
#Category(String) class StringInternationalization {
String вСтроку() {
this.toString()
}
int длина() {
this.length()
}
}
class ApplyMixin {
static {
String.mixin(StringInternationalization)
final helloString = "Привет мир!"
println helloString.вСтроку()
assert helloString.длина() == helloString.length()
}
}
new Main()
This will create 1 Category class for each localised class and one class to apply all mixin transformations(to register all methods.) Also should be faster, then individual closures.
More reading here: http://groovy.codehaus.org/Category+and+Mixin+transformations
I'm playing with Groovy and I wonder, why doesn't this piece of code works?
package test
interface A {
void myMethod()
}
class B implements A {
void myMethod() {
println "No catch"
}
}
B.metaClass.myMethod = {
println "Catch!"
}
(new B()).myMethod()
It prints out No catch, while I expect it to print Catch! instead.
It's a bug in Groovy, there is an open issue in JIRA: Cannot override methods via metaclass that are part of an interface implementation, GROOVY-3493.
Instead of rewriting B.metaClass.myMethod, try following:
B.metaClass.invokeMethod = {String methodName, args ->
println "Catch!"
}
This blog post describes it quite well.
There is a workaround but it only applies to all classes and not specific instances.
metaclass modification BEFORE construction:
interface I {
def doIt()
}
class T implements I {
def doIt() { true }
}
I.metaClass.doIt = { -> false }
T t = new T()
assert !t.doIt()
metaclass modification AFTER construction:
interface I {
def doIt()
}
class T implements I {
def doIt() { true }
}
T t = new T()
// Removing either of the following two lines breaks this
I.metaClass.doIt = { -> false }
t.metaClass.doIt = { -> false }
assert !t.doIt()
I have simple Groovy category class which adds method to String instances:
final class SampleCategory {
static String withBraces(String self) {
"($self)"
}
}
I want to use this category in my unit tests (for example). It looks like this:
class MyTest {
#Test
void shouldDoThis() {
use (SampleCategory) {
assert 'this'.withBraces() == '(this)'
}
}
#Test
void shouldDoThat() {
use (SampleCategory) {
assert 'that'.withBraces() == '(that)'
}
}
}
What I'd like to achieve, however, is ability to specify that category SampleCategory is used in scope of each and every instance method of MyTest so I don't have to specify use(SampleCategory) { ... } in every method.
Is it possible?
You can use mixin to apply the category directly to String's metaClass. Assign null to the metaClass to reset it to groovy defaults. For example:
#Before void setUp() {
String.mixin(SampleCategory)
}
#After void tearDown() {
String.metaClass = null
}
#Test
void shouldDoThat() {
assert 'that'.withBraces() == '(that)'
}
Now you have the option to use extension modules instead of categories:
http://mrhaki.blogspot.se/2013/01/groovy-goodness-adding-extra-methods.html
On the plus side Intellij will recognize the extensions. I've just noticed that it doesn't even need to be a separate module as suggested by the link, just add META-INF/services/org.codehaus.groovy.runtime.ExtensionModule to the project:
# File: src/main/resources/META-INF/services/org.codehaus.groovy.runtime.ExtensionModule
moduleName = module
moduleVersion = 1.0
extensionClasses = SampleExtension
The extension class is pretty much defined like a normal category:
class SampleExtension {
static String withBraces(String self) {
"($self)"
}
}
Can be used like:
def "Sample extension"() {
expect: 'this'.withBraces() == '(this)'
}
If you are using Spock there is a #Use annotation that can be used on the specifications. The drawback with that is that Intellij will not recognize it.