First off, I'm not sure how to best word my solution so if I seem to be babbling at times then please consider this.
There is an interface in a library I wish to modify without touching the physical code,
public interface ProxiedPlayer {
// .. other code
public void setPermission(String permission, boolean state);
}
I have written a third party library for handling permissions and having to hook into my API to edit permissions may be a step some developers do not want to take. So I ask that when setPermission is called is it possible to have it invoke my invoke the appropriate method in my library that will handle permission setting whilst ignoring the pre-programmed code or not?
Here is the full interface I am attempting to proxy.
I have looked into the Java Proxy class but it seems you need an instance of the object you're trying to proxy in the first place. Given that the method can be called any time I do not believe this to be my solution but will happily stand corrected.
I do not have control over instantiation of classes implementing the ProxiedPlayer interface.
EDIT: Ignorant me, there several events that I can subscribe to where it is possible to get an instance of the player, would this be the appropriate place to attempt to proxy the method? One of these events is fired when a player joins the server and getting the instance of the player is possible.
Would the Proxy code need to be called for every instance of the ProxiedPlayer interface or is it possible to simply proxy every invocation of the method in an easier way?
My library is a plugin loaded after everything else that is essential has finished loading.
Edit #2:
import net.md_5.bungee.api.connection.ProxiedPlayer;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class InvocationProxy implements InvocationHandler {
#Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
ProxiedPlayer player = (ProxiedPlayer) proxy;
if(method.getName().equals("setPermission")) {
// Call my code here?
}
return method.invoke(player, args);
}
}
Would something along the lines of what I have above work or am I barking up the wrong tree entirely?
If you do not want to touch the original source, then you only solve this problem by using a Java agent that redefines any class that implements the ProxiedPlayer interface to enforce your security check before calling the actual method. AspectJ together with a load-time-weaving agent was already mentioned as a possible solution for this but you can also implement a pure Java solution using my library Byte Buddy:
public class InterceptionAgent {
public static void premain(String arguments,
Instrumentation instrumentation) {
new AgentBuilder.Default()
.rebase(isSubtypeOf(ProxiedPlayer.class))
.transform(new AgentBuilder.Transformer() {
#Override
public DynamicType.Builder transform(DynamicType.Builder builder) {
return builder.method(named("setPermission"))
.intercept(MethodDelegation.to(MyInterceptor.class)
.andThen(SuperMethodInvocation.INSTANCE));
}
}).installOn(instrumentation);
}
}
With this agent, you more or less specify that you want to redefine any class that is a subtype of ProxiedPlayer to redefine (any) method named setPermisson in order to call a MyInterceptor (that would be your code) and to subsequently call the original implementation.
Note that the suggested implementation assumes that all classes implementing ProxiedPlayer implement the method of this interface and that there is only a single method of this signature. This might be too simple but it shows what direction to go.
Related
Let's say I have 1 complete class with around 20 methods which provide different functionalities.
Now we have multiple clients using this class, but we want them to have restricted access.
For e.g. -
Client 1 - Gets access to method1/m3/m5/m7/m9/m11
Client 2 - Gets access to method2/m4/m6/m8/m10/m12
Is there any way I can restrict this access?
One solution which I thought:
Create 2 new classes extending Parent class and override methods which are not accessible and throw Exception from them.
But then if 3rd client with different requirement, we have to create new subclass for them.
Is there any other way to do this?
Create 2 new classes extending Parent class and override methods which
are not accessible and throw Exception from them. But then if 3rd
client with different requirement, we have to create new subclass for
them.
It is a bad solution because it violates Polymorphism and the Liskov Substitution Principle. This way will make your code less clear.
At first, you should think about your class, are you sure that it isn't overloaded by methods? Are you sure that all of those methods relate to one abstraction? Perhaps, there is a sense to separate methods to different abstractions and classes?
If there is a point in the existence of those methods in the class then you should use different interfaces to different clients. For example, you can make two interfaces for each client
interface InterfaceForClient1 {
public void m1();
public void m3();
public void m5();
public void m7();
public void m9();
public void m11();
}
interface InterfaceForClient2 {
public void m2();
public void m4();
public void m6();
public void m8();
public void m10();
public void m12();
}
And implement them in your class
class MyClass implements InterfaceForClient1, InterfaceForClient2 {
}
After it, clients must use those interfaces instead of the concrete implementation of the class to implement own logic.
You can create an Interface1 which defines methods only for Client1, and an Interface2 which defines methods only for Client2. Then, your class implements Interface1 and Interface2.
When you declare Client1 you can do something like: Interface1 client1.
With this approach, client1 can accesses only methods of this interface.
I hope this will help you.
The other answers already present the idiomatic approach. Another idea is a dynamic proxy decorating the API with an access check.
In essence, you generate a proxy API that has additional checks on method calls to implement a form of Access Control.
Example Implementation:
package com.example;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
#FunctionalInterface
public interface ACL<P, Q> {
boolean allowed(P accessor, Q target, Method method, Object[] args);
class ACLException extends RuntimeException {
ACLException(String message) {
super(message);
}
}
#SuppressWarnings("unchecked")
default Q protect(P accessor, Q delegate, Class<Q> dType) {
if (!dType.isInterface()) {
throw new IllegalArgumentException("Delegate type must be an Interface type");
}
final InvocationHandler handler = (proxy, method, args) -> {
if (allowed(accessor, delegate, method, args)) {
try {
return method.invoke(delegate, args);
} catch (InvocationTargetException e) {
throw e.getCause();
}
} else {
throw new ACLException("Access denies as per ACL");
}
};
return (Q) Proxy.newProxyInstance(dType.getClassLoader(), new Class[]{dType}, handler);
}
}
Example Usage:
package com.example;
import java.lang.reflect.Method;
public class Main {
interface API {
void doAlpha(int arg);
void doBeta(String arg);
void doGamma(Object arg);
}
static class MyAPI implements API {
#Override
public void doAlpha(int arg) {
System.out.println("Alpha");
}
#Override
public void doBeta(String arg) {
System.out.println("Beta");
}
#Override
public void doGamma(Object arg) {
System.out.println("Gamma");
}
}
static class AlphaClient {
void use(API api) {
api.doAlpha(100);
api.doBeta("100");
api.doGamma(this);
}
}
public static class MyACL implements ACL<AlphaClient, API> {
#Override
public boolean allowed(AlphaClient accessor, API target, Method method, Object[] args) {
final String callerName = accessor.getClass().getName().toLowerCase();
final String methodName = method.getName().toLowerCase().replace("do", "");
return callerName.contains(methodName);
}
}
public static void main(String[] args) {
final MyACL acl = new MyACL();
final API api = new MyAPI();
final AlphaClient client = new AlphaClient();
final API guardedAPI = acl.protect(client, api, API.class);
client.use(guardedAPI);
}
}
Notes:
The accessor does not have to be the client object itself, it can be a string key or token that helps ACL identify the client.
The ACL implementation here is rudimentary, more interesting ones could be One that reads ACL from some file or One that uses method and client annotations as rules.
If you don't want to define an interface for API class, consider a tool like javassist to directly proxy a class.
Consider other popular Aspect Oriented Programming solutions
You should create one super class with all the methods and then provide Client specific implementations in their corresponding sub classes extending from the super class defined earlier.
If there are methods which are common implementation for all clients, leave their implementations to the super class.
It seems like you are a bit confused about the purpose of Classes and Interfaces. As far as I know, an Interface is a contract defining which functionality a piece of software provides. This is from official java tutorial:
There are a number of situations in software engineering when it is
important for disparate groups of programmers to agree to a "contract"
that spells out how their software interacts. Each group should be
able to write their code without any knowledge of how the other
group's code is written. Generally speaking, interfaces are such
contracts.
Then you can write a Class which implements this Interface/contract, that is, provides the code that actually perform what was specified. The List interface and the ArrayList class are both an example of this.
Interfaces and Classes have access modifiers, but they aren't designed to specify permissions to specific clients. They specify what is visible for other piece of software depending the location where it is defined: Class, Package, Subclass, World. For example, a private method can be accessed only inside the class where it is defined.
From official Java tutorial again:
Access level modifiers determine whether other classes can use a
particular field or invoke a particular method. There are two levels
of access control:
At the top level—public, or package-private (no explicit modifier).
At the member level—public, private, protected, or package-private (no
explicit modifier).
Maybe you want something more powerful like Access Control List (ACL).
Your question is a little unclear, leading to different possible answers. I'll try to cover some of the possible areas:
Object encapsulation
If your goal is to provide interfaces to different clients that only provide certain functionality or a specific view there are several solutions. Which matches best depends on the purpose of your class:
Refactoring
The question somehow suggests that your class is responsible for different tasks. That might be an indicator, that you could tear it apart into distinct classes that provide the different interfaces.
Original
class AllInOne {
A m1() {}
B m2() {}
C m3() {}
}
client1.useClass(allInOneInstance);
client2.useClass(allInOneInstance);
client3.useClass(allInOneInstance);
Derived
class One {
A m1() {}
}
class Two {
B m2() {}
}
class Three {
C m3() {}
}
client1.useClass(oneInstance);
client2.useClass(twoInstance);
client3.useClass(threeInstance);
Interfaces
If you choose to keep the class together (there might be good reasons for it), you could have the class implement interfaces that model the view required by different clients. By passing instances of the appropriate interface to the clients they will not see the full class interface:
Example
class AllInOne implements I1, I2, I3 {
...
}
interface I1 {
A m1();
}
But be aware that clients will still be able to cast to the full class like ((AllInOne) i1Instance).m2().
Inheritance
This was already outline in other answers. I'll therefore skip this here. I don't think this is a good solution as it might easily break in a lot of scenarios.
Delegation
If casting is a risk to you, you can create classes that only offer the desired interface and delegate to the actual implementation:
Example
class Delegate1 {
private AllInOne allInOne;
public A m1() {
return allInOne.m1();
}
}
Implementing this can be done in various ways and depends on your environment like explicit classes, dynamic proxies , code generation, ...
Framework
If you are using an Application Framework like Spring you might be able to use functionality from this Framework.
Aspects
AOP allows you to intercept method calls and therefor apply some access control logic there.
Security
Please note that all of the above solutions will not give you actual security. Using casts, reflection or other techniques will still allow clients to obtain access to the full functionality.
If you require stronger access limitations there are techniques that I will just briefly outline as they might depend on your environment and are more complex.
Class Loader
Using different class loaders you can make sure that parts of your code have no access to class definitions outsider their scope (used e.g. in tomcat to isolate different deployments).
SecurityManager
Java offers possibilities to implement your own SecurityManager this offers ways to add some extra level of access checking.
Custom build Security
Of course you can add your own access checking logic. Yet I don't think this will be a viable solution for in JVM method access.
As we all know, the self-invokation of bean's method is not working in Spring without AspectJ.
See this question for example.
I think this is because the Spring-created proxy calls the target object's methods using delagate pattern. Like this:
class MyClass {
#Autowired
private MyClass self; // actually a MyProxy instance
#Transactional // or any other proxy magic
public void myMethod() {}
public void myOtherMethod() {
this.myMethod(); // or self.myMethod() to avoid self-invokation problem
}
}
class MyProxy extends MyClass { // or implements MyInterface if proxyMode is not TARGET_CLASS and MyClass also implements MyInterface
private final MyClass delegate;
#Override
public void myMethod() {
// some proxy magic: caching, transaction management etc
delegate.myMethod();
// some proxy magic: caching, transaction management etc
}
#Override
public void myOtherMethod() {
delegate.myOtherMethod();
}
}
Am I right?
With this code:
public void myOtherMethod() {
this.myMethod();
}
this.myMethod() will bypass the proxy (so all #Transactional or #Cacheable magic) because it is just internal delegate's call... So we should inject a MyClass bean (which is actually is MyProxy instance) inside MyClass and call self.myMethod() instead. It is understandable.
But why the proxy is implemented this way?
Why it is not just extends the target class, overriding all public methods and calling super instead of delegate?
Like this:
class MyProxy extends MyClass {
// private final MyClass delegate; // no delegate
#Override
public void myMethod() {
// some proxy magic: caching, transaction management etc
super.myMethod();
// some proxy magic: caching, transaction management etc
}
#Override
public void myOtherMethod() {
super.myOtherMethod();
}
}
It should solve the self-invokation problem, where this.myMethod() bypasses the proxy, because in this case this.myMethod(), invoked from MyClass.myOtherMethod() (we remember that MyClass bean actually is MyProxy instance), will invoke overriden child's method (MyProxy.myMethod()).
So, my main question is why it is not implemented this way?
Your assumption that Spring AOP uses delegation for its proxies is correct. This is also documented.
Using CGLIB, you can theoretically use proxy.invokeSuper() in order to achieve the effect you want, i.e. that self-invocation is registered by the aspect implemented by the proxy's method interceptor (I am using Spring's embedded version of CGLIB here, thus the package names):
package spring.aop;
import org.springframework.cglib.proxy.Enhancer;
import org.springframework.cglib.proxy.MethodInterceptor;
import org.springframework.cglib.proxy.MethodProxy;
import java.lang.reflect.Method;
class SampleClass {
public void x() {
System.out.println("x");
y();
}
public void y() {
System.out.println("y");
}
public static void main(String[] args) {
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(SampleClass.class);
enhancer.setCallback(new MethodInterceptor() {
#Override
public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy)
throws Throwable {
if(method.getDeclaringClass() == Object.class)
return proxy.invokeSuper(obj, args);
System.out.println("Before proxy.invokeSuper " + method.getName());
Object result = proxy.invokeSuper(obj, args);
System.out.println("After proxy.invokeSuper " + method.getName());
return result;
}
});
SampleClass proxy = (SampleClass) enhancer.create();
proxy.x();
}
}
Console log:
Before proxy.invokeSuper x
x
Before proxy.invokeSuper y
y
After proxy.invokeSuper y
After proxy.invokeSuper x
This is exactly what you want. The problem starts, however, when you have several aspects: transactions, logging, whatever else. How do you make sure that they all work together?
Option 1: Each aspect gets its own proxy. This obviously will not work unless you nest the proxies into each other according to aspect precedence. But nesting them into each other means inheritance, i.e. one proxy would have to inherit from the other outside-in. Try proxying a CGLIB proxy, it does not work, you get exceptions. Furthermore, CGLIB proxies are quite expensive and use perm-gen memory, see descriptions in this CGLIB primer.
Option 2: Use composition instead of inheritance. Composition is more flexible. Having one proxy to which you can register aspects as needed solves the inheritance problem, but also means delegation: The proxy registers the aspects and calls their methods during runtime in the right order before/after the actual real object's code is executed (or not, if an #Around advice never calls proceed()). See this example from the Spring manual about manually registering aspects to a proxy:
// create a factory that can generate a proxy for the given target object
AspectJProxyFactory factory = new AspectJProxyFactory(targetObject);
// add an aspect, the class must be an #AspectJ aspect
// you can call this as many times as you need with different aspects
factory.addAspect(SecurityManager.class);
// you can also add existing aspect instances, the type of the object supplied must be an #AspectJ aspect
factory.addAspect(usageTracker);
// now get the proxy object...
MyInterfaceType proxy = factory.getProxy();
As to why the Spring developers chose this approach and whether it might have been possible to use the one-proxy approach but still make sure that self-invocation works like in my little CGLIB sample "logging aspect" above, I can only speculate. You can maybe ask them on the developers mailing list or look into the source code. Maybe the reason was that CGLIB proxies should behave similarly to the default Java dynamic proxies so as to make switching between the two for interface types seamless. Maybe the reason is another one.
I did not mean to be rude in my comments, only straightforward, because your question is really not suited to StackOverflow because it is not a technical problem to which someone can find a solution. It is a historical design question and rather philosophic in nature because with AspectJ a solution to your technical problem (self-invocation) beneath the actual question already exists. But maybe you still want to dive into the Spring source code, change the Spring AOP implementation from delegation to proxy.invokeSuper() and file a pull request. I am not sure such a breaking change would be accepted, though.
In addition, you will not able to use Inheritance + super in the following cases:
What about if the RealSubject is final, so the proxy will can NOT extends it
What about if the Proxy needs to extend something other than the RealSubject
What about if you need to hide some functionality (methods) inside the RealSubject
Prefer Composition over Inheritance (recommended by many developers)
I was reading about default methods in Java 8 and I got stuck in one thing - is there a way to invoke default method from interface without implementing it, or using dynamic Proxy? By using just a simple way, like in following methods:
interface DefaultTestInterface{
default void method1(){
//default method
}
}
class ImplementingClass implements DefaultTestInterface{
public void method1(){
//default method invocation in implementing method
DefaultTestInterface.super.method1();
}
void method2(){
//default method invocation in implementing class
DefaultTestInterface.super.method1();
}
}
public class Main {
public static void main(String[] args) {
//is there any way to simply invoke default method without using proxy and reflection?
}
}
I read similar questions, but the first was connected only with invocation in implementing method, and two others was connected with dynamic Proxy using reflection and reflection.
Those solutions are quite complicated and I am wondering if there is simpler way of doing it. I also read those articles, but I didn't found solution for my problem. I would be grateful for any help.
If the interface has only one method, or all its methods have default implementations, all you need to do is creating an anonymous implementation that does not implement the method that you wish to call:
(new DefaultTestInterface() {}).method1();
Demo.
Is there a way to modify class instance in Java? For example I want to provide custom callback method.
How is it possible to provide separate method/context/scope for each instance of a class?
In javascript it's easy to modify an existing class. It's easy to pass an anonymous function as callback to any method as well. With it's context and scope.
Or do I have to extend my custom class for every request if I need a different callback?
You can do it on instantiation like:
Whatever w = new Whatever("Something") {
public void onWhateverDoThis() {
//...
}
};
So where I personally used it recently were callback methods of a modal:
ModalManager.show(new Modal("Title", "Text") {
public void onDismiss() {
//Do something on dismiss
}
public void onConfirm() {
//Do something on confirm
}
});
Java and Javascript are very different languages. You'll be very frustrated if you try to apply to one a programming paradigm that works in the other. I too was very frustrated with Javascript at the beginning, coming from Java, until I understood the fundamental differences between dynamic and static languages.
In this particular case I don't think it's a good idea to extend your class every time you need to use a different callback, as it would result in a large number of subclasses. If you need a lot of objects that differ only on a particular aspect, wouldn't it be better to have a class or interface that represents that particular aspect?
A possible solution (but not the only correct one by any means) would be to use the Strategy pattern: define an interface that represents a callback and implement the interface for every different callback you need. You can then pass those callbacks as parameters just as you do in Javascript, with the exception that those callbacks won't be able to access any non-public member of the class that calls them.
Also be sure to take a look at the Apache Commons Functor library, which is essentially about having objects that represent functions. I've never used it but, being Apache Commons, it sure would be my first stop if I had your requirements.
You can get frustrated easily, due classic nonfunctional Java is not for those run time goals you have mentioned.
But indeed, you can implement callback in java by using an interface like this:
Ref: CALLBACK PATTERN IN JAVA ENVIRONMENT
For example, suppose you want to be notified when an event happens.
You can define an interface like this:
public interface SomeEvent{
// A regular method, it can return something or take arguments
public void someEventMethod();
}
Then, define a class that will signal/notify the event, It needs to expect objects that implement the SomeEvent interface and then invoke the someEventMethod() method as appropriate.
public class EventNotifier{
private SomeEvent se;
private boolean somethingHappened;
public EventNotifier (SomeEvent event){
//Save the event object for later use.
se = event;
// Nothing to report yet.
somethingHappened = false;
}
//...
public void doWork (){
//Check the predicate, which is set elsewhere.
if (somethingHappened){
//Signal the even by invoking the interface's method
se.someEventMethod();
}
//...
}
// ...
}
Finally, write some code that expects to receive the event notification, it must implement the SomeEvent interface and just pass a reference to itself to the event notifier, like this:
public class CallMe implements SomeEvent{
private EventNotifier en;
public CallMe (){
//Create the event notifier and pass itself to it.
en = new EventNotifier (this);
}
//Define the actual handler for the event
public void someEventMethod(){
// Some event interesting must have occurred
// Do something...
}
}
EDIT 1: By generic I don't mean a generic method for java's generic classes, but a method that I have written to be essential in the use of my program.
I'm trying to write a program (sort of a process integrator) that allows 3rd party developers to add their own functional pieces to a task net. These pieces are objects created from classes which have a runProcess()-method (the class implements specialRunnable).
I wan't to force a log entry to be written whenever the object's runProcess()- method is called. However, I don't want the implementation (writing to log) to be neither in the 3rd party class nor in the class which makes the method call.
I've searched and tried to do it trough inheritance and implementing an interface, but haven't found a solution. Here's and example of how I would like it to work:
public abstract class Process{
public void runProcess(){
// when runProcess() is called all specialized processes write to log first
writeToLog();
// then do their thing which is defined in their class
doYourProcessSpecificThing();
}
public void writeToLog(){
//writing to log comes here
}
// specialized processes have to define what is done
public abstract void doYourProcessSpecificThing();
Specialized class:
public class Special3rdPartyProcess extends Process implements specialRunnable{
runProcess(){
super.runProcess();
}
doYourProcessSpecificThing(){
// this is where the magic happens
}
To sum what I want: I want all processes to be started with runProcess() command, and I want a log entry whenever it is done, but I DON'T want the 3rd party developers to decide how or if the entry is written. Also I don't want it done like this:
writeToLog();
task1.runProcess();
writeToLog();
task2.runProcess
Thanks!
If you make your runProcess method final, then subclasses won't be able to override your method, and this can ensure that writeToLog is called.
You can make writeToLog private to not expose the implementation.
You can make doYourProcessSpecificThing protected so that it can't be called directly, but subclasses can still define their own implementation.
This is called the Template Method Pattern. This allows the implementer (you) to define what specific behavior can be overridden, yet retaining control over the overall process/algorithm.
You can simply make runProcess final in the base class, so subclasses can't override it:
public abstract class Process{
public final void runProcess(){
writeToLog();
doYourProcessSpecificThing();
}
//private: implementation detail
private void writeToLog(){
}
//protected: calling classes don't need to know about this method
protected abstract void doYourProcessSpecificThing();
And your subclass:
public class Special3rdPartyProcess extends Process implements specialRunnable{
protected final void doYourProcessSpecificThing(){
// this is where the magic happens
}
}
Then the client code simply does:
Special3rdPartyProcess spp = ...;
spp.runProcess();