We have some legacy reflected proxy generation code which basically works like this if you look at it as a black box:
Object someObject = new Anything();
Object debugObject = ProxyUtils.wrapWithDebugLogging(someObject);
wrapWithDebugLogging takes any object and overrides any method it can (final methods are obviously unfixable if you're extending a real class), intercepting it to log a message about the call, then calling the real method.
Inside, it's using cglib to do the work and has a bit of protective logic before it constructs the proxy, because anonymous classes are final, yet can be handled by using the superclass or single interface they implement:
Class<?> clazz = someObject.getClass();
Class<?> interfaces = clazz.getInterfaces();
// Anonymous classes are final so you can't extend them, but we know they only have one
// superclass or one interface.
if (clazz.isAnonymousClass()) {
clazz = interfaces.length > 0 ?
interfaces[0] : primaryType.getSuperclass();
}
Enhancer enhancer = new Enhancer();
if (clazz.isInterface()) {
interfaces.add(clazz);
} else {
enhancer.setSuperclass(primaryType);
}
enhancer.setInterfaces(interfaces.toArray(new Class[interfaces.size()]));
The problem is that Java 8's "lambda" classes return false for isAnonymousClass(). But we would like to treat them exactly the same as the anonymous class.
It has been pointed out before that there is no way to determine that a class is a lambda class "by design". But this just seems more like something lacking in the reflection API to me and it certainly isn't the first time Java has "forgotten" to add something obvious to a new API.
So is there a sensible way to distinguish a lambda from a non-lambda without having this feature in the API? I can see that isSynthetic() returns true, but it also returns true for all kinds of other things, presumably.
You shouldn’t create conditional code depending on the question whether a class was generated for a lambda or not. After all, only the properties of the class matter.
The class is final so yo can’t subclass it. Even if it wasn’t final, subclasses weren’t possible due to the fact that it has only private constructors. And it implements an interface. These are the relevant properties of the class.
It’s not unrealistic to encounter the same scenario without any lambda expressions:
final class NotALambda implements Function<String,String> {
public static final Function<String,String> INSTANCE=new NotALambda();
private NotALambda() {}
public String apply(String t) {
return t.toLowerCase();
}
}
Why do you want to treat this class different from a class generated via
Function<String,String> f=String::toLowerCase;? It has the same properties and the same obstacles for creating a proxy. And in the comments you said you want to make a difference based on the question whether the method is declared final or not. This makes even lesser sense as I could add a final modifier to the method in the above example without changing anything, neither the semantic nor the difficulties you will face when creating a proxy.
I don't think this limitation (if you want to call it that) should be an issue.
If you're properly programming to interfaces where appropriate, making your proxy have a superclass (other than Object) becomes unnecessary. Set the proxy's superclass when it is available (no private constructor, not final, etc.).
In all cases, all you need to do is capture the proxied object, the target, intercept all method invocations on it, do your logging, and then delegate or route the invocation to it.
public static Object wrapWithDebugLogging(Object target) {
... // prepare the enhancer as described above
enhancer.setCallback(new MethodInterceptor() {
#Override
public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
logger.debug("Some useful logging message.");
return method.invoke(target, args);
}
});
return enhancer.create();
}
It doesn't matter if the method invoked was final, you're not trying to override it, you're just intercepting it.
Here are the results of a quick experiment. The foo method just outputs getClass().toString() of its argument. (getClass().getName() outputs the same results.)
foo (new Runnable() {
#Override
public void run() {
System.out.println("run");
}
});
output: Test32$1
foo (() -> System.out.println("run"));
output: Test32$$Lambda$1/640070680
No guarantees about how portable this is.
I want to create a private Interface in Class A and have it implemented by Class B. My intention is to have a way for Class A to call a method set on class B that NO ONE else can call. They are in separate file in separate packages. Anyone have any ideas?
The best you can achieve is to give the interface package level visibility and move Class A and B into the same package.
This doesn't stop someone adding another class into the same package in the future, thus giving it access to the interface.
short answer is redesign your class structure.
But if you really need to, consider to use reflex feature in java. and you can inject the method although not recommended.
Disclaimer: not a Java programmer.
But if you want to leverage a type system to get compile-time errors... there are often tricks by introducing a new data type as a sort of "access token" or "dummy parameter". Make it hard to get ahold of a value of that type, but require a value of that type as a parameter in the interface.
Yet introducing a hoop like that winds up being about as contrived as renaming your methods alarming things like DoFooActionOnClassB_ButDontCallUnlessYouAreClassA. I think one usually finds that in a good design, this "dummy type" isn't a dummy type at all... but a capture of the context and state that you should have had in the first place.
I understand that you want to have methods on class B which can only be called from class A. One way would be deferring the real check until runtime but let the compiler make it hard to do the wrong thing. So you could try using a secret which only class A can have in order to protect the method in class B.
public class A {
private static final PrivateA PROOF = new PrivateA();
public static class PrivateA {
private PrivateA() { }
// only A can extend PrivateA
}
public static void main(String[] args) {
new B().methodForAOnly(PROOF, "A");
}
}
Here A's PrivateA is a type which only A can instantiate or extend, and B knows about that...
public class B {
public void methodForAOnly(PrivateA proof, String param) {
if (proof == null) throw new NullPointerException();
// do something
System.out.println(param);
}
}
I looked at an example where delegation pattern is explained for java. Didn't find much use for it (excuse the ignorance) as i feel it lacks the flexibility in objective-c. Is there way to dynamically set the delegate object as one can do in objective-c. Isn't that the whole point of delegation? My knowledge of java is very preliminary , so please explain a bit in detail.
I think there are many ways ways to implement delegation pattern in Java, but probably none which feels like a built-in.
Take a look at the Wikipedia example. Your basic option is to manually implement an interface, and then simply forward the calls to a concrete instance which you can change during run-time as much as you wish.
Now depending on what tools you have and can use, you can make this forwarding more automatic. One idea, is to use aspect-oriented programming, like AspectJ.
Having an aspect compiler (or runtime) you could utilize annotations and come up with a simple extension to the language:
class Person {
#Delegate Animal animal;
...
}
You'd then have to write an aspect that finds #Delegates and automatically adds forwarding methods in the class'es (eg. Person) interface.
If you are able to use a more groovy JVM language, then you wouldn't even have to write a single line of code, because such languages have delegates in the standard library:
You can have a look here, to see how it's done in Groovy. (essentialy exactly like the Person example syntax I came up with... but built-in!)
This is how you fake a functional idiom in Java. Yes, it's hideous. If you want elegant functional idioms, use a language that doesn't treat functions like red-headed step-children. If you want more compile time safety, you can of course use more specific interface definitions so that you don't get IndexOutOfBounds and ClassCastException all over the place from programming errors.
public interface DelegatedFunction<T> {
T call(Object... args);
}
public class DoesSomeDelegatedTask {
private List<DelegatedFunction<String>> delegatedFunctions = new ArrayList<>(1);
public void addFunction(DelegatedFunction<String> function) {
delegatedFunctions.add(function);
}
public void execute() {
for (DelegatedFunction<String> function: delegatedFunctions) {
System.out.println(function(something, someotherthing, whatever));
}
}
}
public class Main {
public static void main(String[] args) throws Exception {
DoesSomeDelegateTask doer = new DoesSomeDelegatedTask();
doer.addFunction(new DelegatedFunction<String> () {
#Override
public String call(Object... args) {
return ((SomeThings) args[0]).whatever((SomeOtherThing) args[1]//you get the idea
}
}
doer.execute();
}
}
Does the Java language have delegate features, similar to how C# has support for delegates?
Not really, no.
You may be able to achieve the same effect by using reflection to get Method objects you can then invoke, and the other way is to create an interface with a single 'invoke' or 'execute' method, and then instantiate them to call the method your interested in (i.e. using an anonymous inner class).
You might also find this article interesting / useful : A Java Programmer Looks at C# Delegates (#blueskyprojects.com)
Depending precisely what you mean, you can achieve a similar effect (passing around a method) using the Strategy Pattern.
Instead of a line like this declaring a named method signature:
// C#
public delegate void SomeFunction();
declare an interface:
// Java
public interface ISomeBehaviour {
void SomeFunction();
}
For concrete implementations of the method, define a class that implements the behaviour:
// Java
public class TypeABehaviour implements ISomeBehaviour {
public void SomeFunction() {
// TypeA behaviour
}
}
public class TypeBBehaviour implements ISomeBehaviour {
public void SomeFunction() {
// TypeB behaviour
}
}
Then wherever you would have had a SomeFunction delegate in C#, use an ISomeBehaviour reference instead:
// C#
SomeFunction doSomething = SomeMethod;
doSomething();
doSomething = SomeOtherMethod;
doSomething();
// Java
ISomeBehaviour someBehaviour = new TypeABehaviour();
someBehaviour.SomeFunction();
someBehaviour = new TypeBBehaviour();
someBehaviour.SomeFunction();
With anonymous inner classes, you can even avoid declaring separate named classes and almost treat them like real delegate functions.
// Java
public void SomeMethod(ISomeBehaviour pSomeBehaviour) {
...
}
...
SomeMethod(new ISomeBehaviour() {
#Override
public void SomeFunction() {
// your implementation
}
});
This should probably only be used when the implementation is very specific to the current context and wouldn't benefit from being reused.
And then of course in Java 8, these do become basically lambda expressions:
// Java 8
SomeMethod(() -> { /* your implementation */ });
Short story: no.
Introduction
The newest version of the Microsoft Visual J++ development environment
supports a language construct called delegates or bound method
references. This construct, and the new keywords delegate and
multicast introduced to support it, are not a part of the JavaTM
programming language, which is specified by the Java Language
Specification and amended by the Inner Classes Specification included
in the documentation for the JDKTM 1.1 software.
It is unlikely that the Java programming language will ever include
this construct. Sun already carefully considered adopting it in 1996,
to the extent of building and discarding working prototypes. Our
conclusion was that bound method references are unnecessary and
detrimental to the language. This decision was made in consultation
with Borland International, who had previous experience with bound
method references in Delphi Object Pascal.
We believe bound method references are unnecessary because another
design alternative, inner classes, provides equal or superior
functionality. In particular, inner classes fully support the
requirements of user-interface event handling, and have been used to
implement a user-interface API at least as comprehensive as the
Windows Foundation Classes.
We believe bound method references are harmful because they detract
from the simplicity of the Java programming language and the
pervasively object-oriented character of the APIs. Bound method
references also introduce irregularity into the language syntax and
scoping rules. Finally, they dilute the investment in VM technologies
because VMs are required to handle additional and disparate types of
references and method linkage efficiently.
Have you read this :
Delegates are a useful construct in event-based systems. Essentially
Delegates are objects that encode a method dispatch on a specified
object. This document shows how java inner classes provide a more
generic solution to such problems.
What is a Delegate? Really it is very similar to a pointer to member
function as used in C++. But a delegate contains the target object
alongwith the method to be invoked. Ideally it would be nice to be
able to say:
obj.registerHandler(ano.methodOne);
..and that the method methodOne would be called on ano when some specific event was received.
This is what the Delegate structure achieves.
Java Inner Classes
It has been argued that Java provides this
functionality via anonymous inner classes and thus does not need the additional
Delegate construct.
obj.registerHandler(new Handler() {
public void handleIt(Event ev) {
methodOne(ev);
}
} );
At first glance this seems correct but at the same time a nuisance.
Because for many event processing examples the simplicity of the
Delegates syntax is very attractive.
General Handler
However, if event-based programming is used in a more
pervasive manner, say, for example, as a part of a general
asynchronous programming environment, there is more at stake.
In such a general situation, it is not sufficient to include only the
target method and target object instance. In general there may be
other parameters required, that are determined within the context when
the event handler is registered.
In this more general situation, the java approach can provide a very
elegant solution, particularly when combined with use of final
variables:
void processState(final T1 p1, final T2 dispatch) {
final int a1 = someCalculation();
m_obj.registerHandler(new Handler() {
public void handleIt(Event ev) {
dispatch.methodOne(a1, ev, p1);
}
} );
}
final * final * final
Got your attention?
Note that the final variables are accessible from within the anonymous
class method definitions. Be sure to study this code carefully to
understand the ramifications. This is potentially a very powerful
technique. For example, it can be used to good effect when registering
handlers in MiniDOM and in more general situations.
By contrast, the Delegate construct does not provide a solution for
this more general requirement, and as such should be rejected as an
idiom on which designs can be based.
I know this post is old, but Java 8 has added lambdas, and the concept of a functional interface, which is any interface with only one method. Together these offer similar functionality to C# delegates. See here for more info, or just google Java Lambdas.
http://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-final.html
No, but they're fakeable using proxies and reflection:
public static class TestClass {
public String knockKnock() {
return "who's there?";
}
}
private final TestClass testInstance = new TestClass();
#Test public void
can_delegate_a_single_method_interface_to_an_instance() throws Exception {
Delegator<TestClass, Callable<String>> knockKnockDelegator = Delegator.ofMethod("knockKnock")
.of(TestClass.class)
.to(Callable.class);
Callable<String> callable = knockKnockDelegator.delegateTo(testInstance);
assertThat(callable.call(), is("who's there?"));
}
The nice thing about this idiom is that you can verify that the delegated-to method exists, and has the required signature, at the point where you create the delegator (although not at compile-time, unfortunately, although a FindBugs plug-in might help here), then use it safely to delegate to various instances.
See the karg code on github for more tests and implementation.
Yes & No, but delegate pattern in Java could be thought of this way. This video tutorial is about data exchange between activity - fragments, and it has great essence of delegate sorta pattern using interfaces.
I have implemented callback/delegate support in Java using reflection. Details and working source are available on my website.
How It Works
There is a principle class named Callback with a nested class named WithParms. The API which needs the callback will take a Callback object as a parameter and, if neccessary, create a Callback.WithParms as a method variable. Since a great many of the applications of this object will be recursive, this works very cleanly.
With performance still a high priority to me, I didn't want to be required to create a throwaway object array to hold the parameters for every invocation - after all in a large data structure there could be thousands of elements, and in a message processing scenario we could end up processing thousands of data structures a second.
In order to be threadsafe the parameter array needs to exist uniquely for each invocation of the API method, and for efficiency the same one should be used for every invocation of the callback; I needed a second object which would be cheap to create in order to bind the callback with a parameter array for invocation. But, in some scenarios, the invoker would already have a the parameter array for other reasons. For these two reasons, the parameter array does not belong in the Callback object. Also the choice of invocation (passing the parameters as an array or as individual objects) belongs in the hands of the API using the callback enabling it to use whichever invocation is best suited to its inner workings.
The WithParms nested class, then, is optional and serves two purposes, it contains the parameter object array needed for the callback invocations, and it provides 10 overloaded invoke() methods (with from 1 to 10 parameters) which load the parameter array and then invoke the callback target.
What follows is an example using a callback to process the files in a directory tree. This is an initial validation pass which just counts the files to process and ensure none exceed a predetermined maximum size. In this case we just create the callback inline with the API invocation. However, we reflect the target method out as a static value so that the reflection is not done every time.
static private final Method COUNT =Callback.getMethod(Xxx.class,"callback_count",true,File.class,File.class);
...
IoUtil.processDirectory(root,new Callback(this,COUNT),selector);
...
private void callback_count(File dir, File fil) {
if(fil!=null) { // file is null for processing a directory
fileTotal++;
if(fil.length()>fileSizeLimit) {
throw new Abort("Failed","File size exceeds maximum of "+TextUtil.formatNumber(fileSizeLimit)+" bytes: "+fil);
}
}
progress("Counting",dir,fileTotal);
}
IoUtil.processDirectory():
/**
* Process a directory using callbacks. To interrupt, the callback must throw an (unchecked) exception.
* Subdirectories are processed only if the selector is null or selects the directories, and are done
* after the files in any given directory. When the callback is invoked for a directory, the file
* argument is null;
* <p>
* The callback signature is:
* <pre> void callback(File dir, File ent);</pre>
* <p>
* #return The number of files processed.
*/
static public int processDirectory(File dir, Callback cbk, FileSelector sel) {
return _processDirectory(dir,new Callback.WithParms(cbk,2),sel);
}
static private int _processDirectory(File dir, Callback.WithParms cbk, FileSelector sel) {
int cnt=0;
if(!dir.isDirectory()) {
if(sel==null || sel.accept(dir)) { cbk.invoke(dir.getParent(),dir); cnt++; }
}
else {
cbk.invoke(dir,(Object[])null);
File[] lst=(sel==null ? dir.listFiles() : dir.listFiles(sel));
if(lst!=null) {
for(int xa=0; xa<lst.length; xa++) {
File ent=lst[xa];
if(!ent.isDirectory()) {
cbk.invoke(dir,ent);
lst[xa]=null;
cnt++;
}
}
for(int xa=0; xa<lst.length; xa++) {
File ent=lst[xa];
if(ent!=null) { cnt+=_processDirectory(ent,cbk,sel); }
}
}
}
return cnt;
}
This example illustrates the beauty of this approach - the application specific logic is abstracted into the callback, and the drudgery of recursively walking a directory tree is tucked nicely away in a completely reusable static utility method. And we don't have to repeatedly pay the price of defining and implementing an interface for every new use. Of course, the argument for an interface is that it is far more explicit about what to implement (it's enforced, not simply documented) - but in practice I have not found it to be a problem to get the callback definition right.
Defining and implementing an interface is not really so bad (unless you're distributing applets, as I am, where avoiding creating extra classes actually matters), but where this really shines is when you have multiple callbacks in a single class. Not only is being forced to push them each into a separate inner class added overhead in the deployed application, but it's downright tedious to program and all that boiler-plate code is really just "noise".
It doesn't have an explicit delegate keyword as C#, but you can achieve similar in Java 8 by using a functional interface (i.e. any interface with exactly one method) and lambda:
private interface SingleFunc {
void printMe();
}
public static void main(String[] args) {
SingleFunc sf = () -> {
System.out.println("Hello, I am a simple single func.");
};
SingleFunc sfComplex = () -> {
System.out.println("Hello, I am a COMPLEX single func.");
};
delegate(sf);
delegate(sfComplex);
}
private static void delegate(SingleFunc f) {
f.printMe();
}
Every new object of type SingleFunc must implement printMe(), so it is safe to pass it to another method (e.g. delegate(SingleFunc)) to call the printMe() method.
With safety-mirror on the classpath you get something similar to C#'s delegates and events.
Examples from the project's README:
Delegates in Java!
Delegate.With1Param<String, String> greetingsDelegate = new Delegate.With1Param<>();
greetingsDelegate.add(str -> "Hello " + str);
greetingsDelegate.add(str -> "Goodbye " + str);
DelegateInvocationResult<String> invocationResult =
greetingsDelegate.invokeAndAggregateExceptions("Sir");
invocationResult.getFunctionInvocationResults().forEach(funInvRes ->
System.out.println(funInvRes.getResult()));
//prints: "Hello sir" and "Goodbye Sir"
Events
//Create a private Delegate. Make sure it is private so only *you* can invoke it.
private static Delegate.With0Params<String> trimDelegate = new Delegate.With0Params<>();
//Create a public Event using the delegate you just created.
public static Event.With0Params<String> trimEvent= new Event.With0Params<>(trimDelegate)
See also this SO answer.
While it is nowhere nearly as clean, but you could implement something like C# delegates using a Java Proxy.
No, but it has similar behavior, internally.
In C# delegates are used to creates a separate entry point and they work much like a function pointer.
In java there is no thing as function pointer (on a upper look) but internally Java needs to do the same thing in order to achieve these objectives.
For example, creating threads in Java requires a class extending Thread or implementing Runnable, because a class object variable can be used a memory location pointer.
No, Java doesn't have that amazing feature. But you could create it manually using the observer pattern. Here is an example:
Write C# delegate in java
The code described offers many of the advantages of C# delegates. Methods, either static or dynamic, can be treated in a uniform manner. The complexity in calling methods through reflection is reduced and the code is reusable, in the sense of requiring no additional classes in the user code. Note we are calling an alternate convenience version of invoke, where a method with one parameter can be called without creating an object array.Java code below:
class Class1 {
public void show(String s) { System.out.println(s); }
}
class Class2 {
public void display(String s) { System.out.println(s); }
}
// allows static method as well
class Class3 {
public static void staticDisplay(String s) { System.out.println(s); }
}
public class TestDelegate {
public static final Class[] OUTPUT_ARGS = { String.class };
public final Delegator DO_SHOW = new Delegator(OUTPUT_ARGS,Void.TYPE);
public void main(String[] args) {
Delegate[] items = new Delegate[3];
items[0] = DO_SHOW .build(new Class1(),"show,);
items[1] = DO_SHOW.build (new Class2(),"display");
items[2] = DO_SHOW.build(Class3.class, "staticDisplay");
for(int i = 0; i < items.length; i++) {
items[i].invoke("Hello World");
}
}
}
Java doesn't have delegates and is proud of it :). From what I read here I found in essence 2 ways to fake delegates:
1. reflection;
2. inner class
Reflections are slooooow! Inner class does not cover the simplest use-case: sort function. Do not want to go into details, but the solution with inner class basically is to create a wrapper class for an array of integers to be sorted in ascending order and an class for an array of integers to be sorted in descending order.
Is there anyway to override a method at run time? Even if it requires dynamically creating a subclass from that instance?
With plain Java, no.
With ByteBuddy(preferred), asm, cglib or aspectj, yes.
In plain Java, the thing to do in a situation like that is to create an interface-based proxy that handles the method invocation and delegates to the original object (or not).
You could create an anonymous class that overrides the method and uses the strategy pattern to decide what to do.
If you are looking for dynamic compilation from code, you can follow these instructions
As others said, no, you can't override a method at runtime. However, starting with Java 8 you can take the functional approach. Function is a functional interface that allows you to treat functions as reference types. This means that you can create several ones and switch between them (dynamically) a-la strategy pattern.
Let's look at an example:
public class Example {
Function<Integer, Integer> calculateFuntion;
public Example() {
calculateFuntion = input -> input + 1;
System.out.println(calculate(10));
// all sorts of things happen
calculateFuntion = input -> input - 1;
System.out.println(calculate(10));
}
public int calculate(int input) {
return calculateFuntion.apply(input);
}
public static void main(String[] args) {
new Example();
}
}
Output:
11
9
I don't know under what circumstances and design you intend to override, but the point is that you replace the behavior of the method, which is what overriding does.
I think it not possible with simple Java.
With reflection and/or cglib probally you can do it.
Look at these links:
http://www.rgagnon.com/javadetails/java-0039.html
http://www.javaworld.com/javaworld/jw-06-2006/jw-0612-dynamic.html