Groovy adds each() and a number of other methods to java.lang.Object. I can't figure out how to use the Groovy metaclass to dynamically replace the default each() on a Java class.
I can see how to add new methods:
MyJavaClass.metaClass.myNewMethod = { closure -> /* custom logic */ }
new MyJavaClass().myNewMethod { item -> println item } // runs custom logic
But it seems the same approach doesn't work when overriding methods:
MyJavaClass.metaClass.each = { closure -> /* custom logic */ }
new MyJavaClass().each { item -> println item } // runs Object.each()
What am I doing wrong? How can I dynamically override each() in Groovy?
Well I found the solution seconds after posting the question. I just needed to explicitly specify the type of the Closure argument on each():
MyJavaClass.metaClass.each = { Closure closure -> /* custom logic */ }
new MyJavaClass().each { item -> println item } // runs custom logic
By leaving out the type, I was adding a more generic overloaded version of each() which accepts an Object argument, rather than overriding the existing each() which accepts a Closure argument.
Related
The code snippet is from the book < Groovy in action 2nd >, with minor modifications.
1 this code works as expected
package test
class InspectMe {
int outer(){
return inner()
}
int inner(){
return 1
}
}
def tracer = new TracingInterceptor(writer: new StringWriter())
def proxyMetaClass = ProxyMetaClass.getInstance(InspectMe)
proxyMetaClass.interceptor = tracer
InspectMe inspectMe = new InspectMe()
inspectMe.metaClass = proxyMetaClass
inspectMe.outer()
println(tracer.writer.toString())
output:
before test.InspectMe.outer()
before test.InspectMe.inner()
after test.InspectMe.inner()
after test.InspectMe.outer()
2 but this code's output is different
package test
class InspectMe {
int outer(){
return inner()
}
int inner(){
return 1
}
}
def tracer = new TracingInterceptor(writer: new StringWriter())
def proxyMetaClass = ProxyMetaClass.getInstance(InspectMe)
proxyMetaClass.interceptor = tracer
InspectMe inspectMe = new InspectMe()
proxyMetaClass.use(inspectMe){
inspectMe.outer()
}
println(tracer.writer.toString())
output:
before test.InspectMe.outer()
after test.InspectMe.outer()
It seems TracingInterceptor dosen't intercept inner methods in the second code.
Maybe it's normal behavior, But it seems to me like a bug.
Can somebody please explain this?
I don't know if this is a bug or not, but I can explain why this different behavior happens. Let's start with analyzing what InspectMe.outer() method implementation looks like at the bytecode level (we decompile .class file):
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
import groovy.lang.GroovyObject;
import groovy.lang.MetaClass;
import org.codehaus.groovy.runtime.BytecodeInterface8;
import org.codehaus.groovy.runtime.callsite.CallSite;
import org.codehaus.groovy.runtime.typehandling.DefaultTypeTransformation;
public class InspectMe implements GroovyObject {
public InspectMe() {
CallSite[] var1 = $getCallSiteArray();
MetaClass var2 = this.$getStaticMetaClass();
this.metaClass = var2;
}
public int outer() {
CallSite[] var1 = $getCallSiteArray();
return !__$stMC && !BytecodeInterface8.disabledStandardMetaClass() ? this.inner() : DefaultTypeTransformation.intUnbox(var1[0].callCurrent(this));
}
public int inner() {
CallSite[] var1 = $getCallSiteArray();
return 1;
}
}
As you can see, the outer() method tests the following predicate
!__$stMC && !BytecodeInterface8.disabledStandardMetaClass()
and if it evaluates to true, it invokes directly this.inner() method avoiding Groovy's MOP (meta-object protocol) layer (no metaclass involved in this case). Otherwise, it invokes var1[0].callCurrent(this) which means that inner() method gets invoked through Groovy's MOP with metaclass and interceptor involved in its execution.
The two examples you have shown in the question present a different way of setting metaclass field. In the first case:
def tracer = new TracingInterceptor(writer: new StringWriter())
def proxyMetaClass = ProxyMetaClass.getInstance(InspectMe)
proxyMetaClass.interceptor = tracer
InspectMe inspectMe = new InspectMe()
inspectMe.metaClass = proxyMetaClass // <-- setting metaClass with DefaultGroovyMethods
inspectMe.outer()
println(tracer.writer.toString())
we are invoking inspectMe.setMetaClass(proxyMetaClass) method using Groovy's MOP layer. This method gets added to InspectMe class by DefaultGroovyMethods.setMetaClass(GroovyObject self, MetaClass metaClass).
Now, if we take a quick look at how this setMetaClass method is implemented we will find something interesting:
/**
* Set the metaclass for a GroovyObject.
* #param self the object whose metaclass we want to set
* #param metaClass the new metaclass value
* #since 2.0.0
*/
public static void setMetaClass(GroovyObject self, MetaClass metaClass) {
// this method was introduced as to prevent from a stack overflow, described in GROOVY-5285
if (metaClass instanceof HandleMetaClass)
metaClass = ((HandleMetaClass)metaClass).getAdaptee();
self.setMetaClass(metaClass);
disablePrimitiveOptimization(self);
}
private static void disablePrimitiveOptimization(Object self) {
Field sdyn;
Class c = self.getClass();
try {
sdyn = c.getDeclaredField(Verifier.STATIC_METACLASS_BOOL);
sdyn.setBoolean(null, true);
} catch (Throwable e) {
//DO NOTHING
}
}
It invokes at the end private method disablePrimitiveOptimization(self). This method is responsible for assigning true to __$stMC class field (the constant Verifier.STATIC_METACLASS_BOOL stores __$stMC value). What does it mean in our case? It means that the predicate in outer() method:
return !__$stMC && !BytecodeInterface8.disabledStandardMetaClass() ? this.inner() : DefaultTypeTransformation.intUnbox(var1[0].callCurrent(this));
evaluates to false, because __$stMC is set to true. And in this case inner() method gets executed via MOP with metaClass and interceptor.
OK, but it explains the first case that works as expected. What happens in the second case?
def tracer = new TracingInterceptor(writer: new StringWriter())
def proxyMetaClass = ProxyMetaClass.getInstance(InspectMe)
proxyMetaClass.interceptor = tracer
InspectMe inspectMe = new InspectMe()
proxyMetaClass.use(inspectMe){
inspectMe.outer()
}
println(tracer.writer.toString())
Firstly, we need to check what does proxyMetaClass.use() look like:
/**
* Use the ProxyMetaClass for the given Closure.
* Cares for balanced setting/unsetting ProxyMetaClass.
*
* #param closure piece of code to be executed with ProxyMetaClass
*/
public Object use(GroovyObject object, Closure closure) {
// grab existing meta (usually adaptee but we may have nested use calls)
MetaClass origMetaClass = object.getMetaClass();
object.setMetaClass(this);
try {
return closure.call();
} finally {
object.setMetaClass(origMetaClass);
}
}
It's pretty simple - it replaces metaClass for the time of closure execution and it sets the old metaClass back when closure's execution completes. Sounds like something similar to the first case, right? Not necessarily. This is Java code and it invokes object.setMetaClass(this) method directly (the object variable is of type GroovyObject which contains setMetaClass method). It means that the field __$stMC is not set to true (the default value is false), so the predicate in outer() method has to evaluate:
BytecodeInterface8.disabledStandardMetaClass()
If we run the second example we will see that this method call returns false:
And that is why the whole expression
!__$stMC && !BytecodeInterface8.disabledStandardMetaClass()
evaluates to true and the branch that invokes this.inner() directly gets executed.
Conclusion
I don't know if it was intended or not, but as you can see dynamic setMetaClass method disables primitive optimizations and continues using MOP, while ProxyMetaClass.use() sets the metaClass keeping primitive optimizations enabled and caused a direct method call. I guess this example shows a corner case no one thought about when implementing ProxyMetaClass class.
UPDATE
It seems like the difference between these two methods exists because ProxyMetaClass.use() was implemented in 2005 for Groovy 1.x and it got updated for the last time in 2009. This __$stMC field was added in 2011 and the DefaultGroovyMethods.setMetaClass(GroovyObject object, Closure cl) was introduced in 2012 according to its javadoc that says this method is available since Groovy 2.0.
In Kotlin, there is the apply method:
inline fun <T> T.apply(block: T.() -> Unit): T (source)
Calls the specified function block with this value as its receiver and returns this value.
This allows you to configure an object like the following:
val myObject = MyObject().apply {
someProperty = "this value"
myMethod()
}
myObject would be the MyObject after the apply {} call.
Groovy has the with method, which is similar:
public static <T,U> T with(U self,
#DelegatesTo(value=DelegatesTo.Target.class,target="self",strategy=1)
Closure<T> closure
)
Allows the closure to be called for the object reference self.
...
And an example from the doc:
def b = new StringBuilder().with {
append('foo')
append('bar')
return it
}
assert b.toString() == 'foobar'
The part with the Groovy method is always having to use return it to return the delegate of the with call, which makes the code considerably more verbose.
Is there an equivalent to the Kotlin apply in Groovy?
The function is called tap and is part of Groovy 2.5. See discussions about the naming in merge request.
Other than that, only foo.with{ bar=baz; it } can be used. You can retrofit your own doto, tap, apply, ... via metaprogramming.
Starting to grasp closures in general and some groovy features.
Given the following code:
class Mailer {
void to(final String to) { println "to $to" }
void from(final String from) { println "from $from" }
static void send(Closure configuration) {
Mailer mailer = new Mailer()
mailer.with configuration
}
}
class MailSender {
static void sendMessage() {
Mailer.send {
to 'them'
from 'me'
}
}
}
MailSender.sendMessage()
What happens under the hood when you pass a closure to Mailer.send method?
Does to and from are passed as arguments from the Closure point of view? Which types the Closure maps them?
And then inside the Mailer.send method at the moment the Mailer object calls mailer.with receiving the configuration object, the object maps them into method calls. Groovy does this by reflection?
Groovy can dynamically define the delegate of a closure and even the this object.
with is setting the delegate and executing the closure. This is a verbose way to achieve the same:
def math = {
given 4
sum 5
print
}
class PrintMath {
def initial
def given(val) {
initial = val
}
def sum(val) {
initial += val
}
def getPrint() {
println initial
return initial
}
}
math.delegate = new PrintMath()
math.resolveStrategy = Closure.DELEGATE_ONLY
assert math() == 9
What happens under the hood when you pass a closure to Mailer.send method?
It receives a not-yet-executed block of code.
Does to and from are passed as arguments from the Closure point of view?
No, it is better thinking of them as an anonymous class/lambda in java, or a function(){} in javascript.
Which types the Closure maps them?
None, they are method calls waiting to be executed. They can be delegated to different objects, though.
And then inside the Mailer.send method at the moment the Mailer object calls mailer.with receiving the configuration object, the object maps them into method calls. Groovy does this by reflection?
You can decompile a Groovy class file to see what is going on. IIRC, Groovy currently uses a "reflector" strategy (with an arrayOfCallSite caching) to make calls faster OR it can use invokedynamic.
The closure math in the code above will result in this class:
// .. a lot of techno-babble
public Object doCall(Object it) {
CallSite[] arrayOfCallSite = $getCallSiteArray();
arrayOfCallSite[0].callCurrent(this, Integer.valueOf(4));
arrayOfCallSite[1].callCurrent(this, Integer.valueOf(5));
return arrayOfCallSite[2].callGroovyObjectGetProperty(this);
return null;
}
I've been trying to create a TEMPORARY override on new objects, and then to remove the override on the objects themselves. I'm not sure if this can be done, but here is what I've tried so far.
// Say I have a class like:
class Validator {
boolean validate() { println "code here to return actual true/false"; false }
}
// I have two integration points one of them is Here before construction:
// First integration point:
// Save actual validate function
def realValidate = Validator.&validate
// Make new instances of Validator have the validate function hardwired to true
Validator.metaClass.validate { -> println "hardwired true"; true }
// Code I'd rather not modify
// Now some code executes which news up an instance and calls validate
def validator = new Validator()
validator.validate() // This correctly calls our override
// Second integration point.
// Without newing up a new Validator object, I'd like to remove the override.
Validator.metaClass = null
validator.metaClass.validate = Validator.&validate
// This throws "java.lang.IllegalArgumentException: object is not an instance of declaring class"
//validator.validate()
// So maybe I have to explicitly say:
realValidate.resolveStrategy = Closure.DELEGATE_FIRST
// But this still throws the same exception
//validator.validate()
// Perhaps if I tell my objects metaclass to forget about validate, it will bubble up and look for the method on its declaring class?
validator.metaClass.validate = { -> throw new MissingMethodException("validate", Validator.class, (Object[])[], false) }
// This throws MissingMethodException: No signature of method: Validator.validate() is applicable for argument types: () values: []
// Possible solutions: validate(), wait()
//validator.validate()
Apologies for not having a super specific question, since I don't know what all is possible in this particular area. I'd love both the reason why my code doesn't work, as well as alternatives to make it work.
This could be a per instance meta class problem... Validator.metaClass = null will set the global meta class for the Validator class to default. but your validator instance here is a Groovy class and thus stores a separate reference to the meta class in the instance itself. Calls with that instance will not go through a lookup of the global meta class and instead use the per instance meta class (the reference stored in the instance itself). Thus validator.metaClass = null is the only way to reset this
A small modification to your strategy would be fruitful. Use metaClass on the object instead of the Class.
// Say I have a class like:
class Validator {
boolean validate() { println "code here to return actual true/false"; false }
}
def validator = new Validator()
// mark that the pointer is on object instead of class
def realValidate = validator.&validate
validator.metaClass.validate { -> println "hardwired true"; true }
validator.validate() // This correctly calls our override
// Second integration point.
// DO NOT NEED THIS
// validator.metaClass = null
// Assign the method pointer to validate to call original validate
validator.metaClass.validate = realValidate
validator.validate()
Your approach did not work because you had validate() overridden on the metaClass of Class reference instead of the object itself.
Is there a way in Groovy that I can add code to a constructor when a class is instantiated? I have a Groovy class (but I can't modify the source of this particular one), but I was hoping there was a way to inject code (maybe via the metaclass) so my code gets run as part of the constructor (in this case there is only one, default constructor).
Thanks,
Jeff
You can override the constructor, but it's a little tricky, particularly if you're overriding the default constructor. You need to assign a closure to the class's metaClass.constructor, and the closure should return a new instance. The tricky part is that if you call the constructor you've overriden, you'll get into a recursive loop and generate a stack overflow. You need another way to get an instance of the class, such as a different constructor.
For testing, it's sometimes possible to get around this limitation. Usually, it's enough to first instantiate an object, then override the constructor to return the existing instance. Example:
class MyObject {
String something
MyObject() { something = "initialized" }
}
testInstance = new MyObject()
testInstance.something = "overriden"
MyObject.metaClass.constructor = { -> testInstance }
aNewObject = new MyObject()
assert aNewObject.is(testInstance)
assert aNewObject.something == "overriden"
It is possible to add new constructors or replace the old one. If you need the original constructor, you can use reflection for that:
MyObject.metaClass.constructor = { -> // for the no-arg ctor
// use reflection to get the original constructor
def constructor = MyObject.class.getConstructor()
// create the new instance
def instance = constructor.newInstance()
// ... do some further stuff with the instance ...
println "Created ${instance}"
instance
}
Note that you have to change this if you have parameters to your constructors, e.g:
// Note that the closure contains the signature of the constructor
MyObject.metaClass.constructor = { int year, String reason ->
def constructor = MyObject.class.getConstructor(Integer.TYPE, String.class)
def instance = constructor.newInstance(
2014, "Boy, am I really answering a question three years old?")
// ... do some further stuff with the instance ...
println "Created ${instance}"
instance
}
PS: Note that when you want to add constructors which are not yet existent, use the << operator instead: MyObject.metaClass.constructor << { /* as above */ }.
You can bypass the limitations in the solution proposed by storing the original constructor using standard Java reflection. For example, this is what I do initialize a class (basic injection) in a spock test:
def setupSpec() {
MockPlexusContainer mockPlexusContainer = new MockPlexusContainer()
def oldConstructor = MY_CLASS.constructors[0]
MY_CLASS.metaClass.constructor = { ->
def mojo = oldConstructor.newInstance()
mockPlexusContainer.initializeContext(mojo)
return mojo
}
}
This gets invoked only once, but eveytime someone calls a constructor I get a different instance avoiding cleaning values and ensuring thread safety.