Java check if has method

Check if the method is invoked

Solution 1: There is no way to do exactly what you said (check whether other threads are inside of a method directly). Well.. I ‘ve downloaded source code, find class and started to looking for method , but there isn’t any method there neither in class, nor in project!

Check if the method is invoked

I am trying to create a logging OSGi bundle, which would be able to log input, output, exception (if any) parameters of all the methods which are marked with specific annotation.

I have tried to set up the Spring AOP logging in OSGi but I was unsuccessful.

All I need to know is how to determine when the method is being called? For example I have scanned all methods in a class which are annotated with my annotation and I have those methods in array or list.

For example there are three methods: getStatus , getDetails , getSomething . And the user invokes getDetails method, so how should I know when this method is invoked?

Detecting method calls requires you to instrument your code. If you want to do this at runtime this requires byte-code manipulation (which is what Spring AOP does, as far as I know). Alternatively you could instrument your code at compile-time using a custom preprocessing step that generates the instrumented Java-code , but I don’t know if that is much easier.

I guess your best bet is to try and Spring AOP working.

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How do I check the method state in java?

Lets say I have a instance method named myMethod(). I need to check if any thread is executing the method. Is there a way I can check this using reflection or some other API?

I want to execute some cleanup logic when the method is NOT in execution. Hence need to check if the method is running or not.

There is no way to do exactly what you said (check whether other threads are inside of a method directly).

However, this sounds like a classic case for synchronization. Make the method invocation synchronize on some object’s monitor; make your cleanup code synchronize on the same monitor, and voila! Now, if any thread is running the method(s) that you’ve made synchronized, your cleanup code can’t run, but will wait until all threads have exited the methods and will then proceed.

public class Foo < private final Object LOCK = new Object(); public void importantMethodOne() < synchronized(LOCK) < // Method body here >> public void importantMethodTwo() < synchronized(LOCK) < // Method body here >> public void otherMethod() < // Method body here >public void cleanupMethod() < synchronized(LOCK) < // Method body here >> > 

The LOCK monitor is a mutex — only one thread can hold it at once, so only one thread can be calling any of importantMethodOne, importantMethodTwo or the cleanupMethod at once. Any threads that try to invoke any of these methods while another is within the scope (and thus has the lock) will wait. In this example, otherMethod can be called at any time without interrupting anything else, because it doesn’t acquire the lock.

Note that in this case, two threads cannot execute myMethod() at once because they’ll block each other . If this is an issue, you can get around this with a readwritelock (which allows many «readers» to hold it at once but one «writer»). If you acquire the read lock for your «normal» methods and the write lock for your «cleanup» methods, this will have the desired semantics.

I suppose you could have something like this: (Use indentation, don’t feel like dealing with braces today)

int busy; public boolean is Running() synchronized(lock1) return busy != 0; public void run() try < synchronized(lock1) busy++ do lots of stuff here >finally

It’s early, but my tired brain can’t find a problem with this—I’ll re-read it later and see if it still makes sense.

But you will need more synchronization logic to ensure that nothing enters it while you are cleaning up—I didn’t code that since today I’m answering the question you asked and not the one you meant to ask (others already did that)

C# Generics and Type Checking, I have a method that uses an IList as a parameter. I need to check what the type of that T object is and do something based on it. I was trying to use the T value, but the compiler does not not allow it. My solution is the following:

How to check whether a method exists in Python?

In the function __getattr__() , if a referred variable is not found then it gives an error. How can I check to see if a variable or method exists as part of an object?

import string import logging class Dynamo: def __init__(self,x): print "In Init def" self.**** def __repr__(self): print self.x def __str__(self): print self.x def __int__(self): print "In Init def" def __getattr__(self, key): print "In getattr" if key == 'color': return 'PapayaWhip' else: raise AttributeError dyn = Dynamo('1') print dyn.color dyn.color = 'LemonChiffon' print dyn.color dyn.__int__() dyn.mymethod() //How to check whether this exist or not 

Check if class has such method?

hasattr(Dynamo, key) and callable(getattr(Dynamo, key)) 

hasattr(Dynamo, 'mymethod') and callable(getattr(Dynamo, 'mymethod')) 

You can use self.__class__ instead of Dynamo

It’s easier to ask forgiveness than to ask permission.

Don’t check to see if a method exists. Don’t waste a single line of code on «checking»

try: dyn.mymethod() # How to check whether this exists or not # Method exists and was used. except AttributeError: # Method does not exist; What now? 

How about dir() function before getattr() ?

I use below utility function. It works on lambda, class methods as well as instance methods.

Utility Method

def has_method(o, name): return callable(getattr(o, name, None)) 

Example Usage

class MyTest: b = 'hello' f = lambda **** @classmethod def fs(): pass def fi(self): pass 

>>> a = MyTest() >>> has_method(a, 'b') False >>> has_method(a, 'f') True >>> has_method(a, 'fs') True >>> has_method(a, 'fi') True >>> has_method(a, 'not_exist') False 

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Check the method in project

I am trying to debug code from https://nunrar.codeplex.com/. In branch DOCUMENTATION -> Basic Usage there is the following code:

RarArchive archive = RarArchive.Open(source); foreach (RarArchiveEntry entry in archive.Entries)

Well.. I ‘ve downloaded source code, find RarArchiveEntry class and started to looking for method ExtractToFile() , but there isn’t any method there neither in class, nor in project! Please help me to find this method or method which i can call to look through how this programm can unpack rar files step-by-step?

It looks like the documentation is obsolete and the name of the method was changed, there is an extension method found in rararchive Entry.Exensions.cs called WriteToFile .

/// 
/// Extract to specific file ///
 public static void WriteToFile(this RarArchiveEntry entry, string destinationFileName, ExtractOptions options = ExtractOptions.Overwrite)

How To Check If A Method Exists At Runtime In Java?, Roland Illig is correct, but wanted to add in an example of how to check if a method exists that requires a parameter using Class.getMethod. Can also use Class.getDeclaredMethod if you are trying to access a private method.

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Check if object has method in Java?

A better idea would be to create an interface with that method and make that the parameter type. Every object passed to your method will have to implement that interface.

It’s called expressing your contract with clients. If you require that method, say so.

Solution 2

Get the instance’s class with Object.getClass() , then use Class.getMethod(String, Class. ) and catch a NoSuchMethodException if the class doesn’t have that method.

On the other hand, this is not a very good practice in Java. Try using interfaces instead.

Solution 3

You can do this using Java reflection.

Something like object.getClass().getMethods() to get an array of the Method objects, or you can use the getMethod() to get an object with a particular name (you have to define the parameters).

Solution 4

First, define an interface that will require all objects implementing it to implement a setAlpha() method:

public interface AlphaChanger < public void setAlpha(int alpha); // concrete classes must implement this >

Then, define classes that implement this interface:

public class Thing implements AlphaChanger < public void setAlpha(int alpha) < // must be present // implementation >> 

Now you can require that all objects passed to your method must implement the AlphaChanger interface:

public void yourMethod(AlphaChanger changer) < changer.setAlpha(10); // you know for sure the method exists >

Connor Deckers

Connor is a Software Developer specialising in Javascript, having developed several in-house toolings and projects. These include a major Chrome browser extension that augments our internal LMS, and several supporting toolsets for in-house use.

Comments

I’m not all too greatly experienced with Java, I know enough to get me through at the moment, but I’m not perfect yet, so I apologize if this is a dumb question. I’m trying to write a class that can have any sort of object passed to it, but I need to check if the object passed has a particular method with it. How would I go about testing this? I hope this makes sense. Cheers. EDIT Thanks for all the fast replies! I’m not too familiar with interfaces etc, so I’m not entirely sure how to use them. But, to give a better insight into what I’m doing, I’m trying to create a class that will affect the alpha of an object, e.g. ImageView or a TextView for instance. How would I create an interface for that, without listing each object individually, when all I need is to make sure that they have the method .setAlpha() ? Does this make sense? Cheers.

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Check if object has method in Java?

I’m not all too greatly experienced with Java, I know enough to get me through at the moment, but I’m not perfect yet, so I apologize if this is a dumb question.

I’m trying to write a class that can have any sort of object passed to it, but I need to check if the object passed has a particular method with it. How would I go about testing this?

I hope this makes sense. Cheers.

Thanks for all the fast replies! I’m not too familiar with interfaces etc, so I’m not entirely sure how to use them. But, to give a better insight into what I’m doing, I’m trying to create a class that will affect the alpha of an object, e.g. ImageView or a TextView for instance. How would I create an interface for that, without listing each object individually, when all I need is to make sure that they have the method .setAlpha() ? Does this make sense? Cheers.

Solution

A better idea would be to create an interface with that method and make that the parameter type. Every object passed to your method will have to implement that interface.

It’s called expressing your contract with clients. If you require that method, say so.

OTHER TIPS

Get the instance’s class with Object.getClass() , then use Class.getMethod(String, Class. ) and catch a NoSuchMethodException if the class doesn’t have that method.

On the other hand, this is not a very good practice in Java. Try using interfaces instead.

You can do this using Java reflection.

Something like object.getClass().getMethods() to get an array of the Method objects, or you can use the getMethod() to get an object with a particular name (you have to define the parameters).

First, define an interface that will require all objects implementing it to implement a setAlpha() method:

public interface AlphaChanger < public void setAlpha(int alpha); // concrete classes must implement this >

Then, define classes that implement this interface:

public class Thing implements AlphaChanger < public void setAlpha(int alpha) < // must be present // implementation >> 

Now you can require that all objects passed to your method must implement the AlphaChanger interface:

public void yourMethod(AlphaChanger changer) < changer.setAlpha(10); // you know for sure the method exists >

You can try and list the classes’ methods like this (example : FooBar class):

Class fooBar= FooBar.class; Method[] methods = fooBar.getDeclaredMethods(); if (methods.length == 0) < . >else

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