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What is Inversion of Control?
I'm really confused with the concept of dependency injection.
I'm very new to software field and I have a doubt on the following scenario. Let say I need a Json parser class for my java code, So I'm adding the Json jar when executing the java program using the classpath argument. My program is dependent on Json jar, so that it mean I'm doing a dependency injection here?
An another example will be, using import statement does solve the problem of user class is depending on other class ( like Date in java ) so all these are the concept of dependency injection?
Where I'm making the concept.
Thanks in advance.
It's much more to do with how you link your components together. e.g. in the above, I would expect you to inject your parser into the class that needs it. e.g.
Instead of:
public class MyParserUsingClass {
...
public MyParserUsingClass() {
this.parser = new Parser();
}
}
you would do:
public class MyParserUsingClass {
...
public MyParserUsingClass(Parser injectedParser) {
this.parser = injectedParser;
}
}
Why do this ? Your class using the parser doesn't really care where the parser comes from and should really use an interface rather than a concrete instance. By injecting the parser, you can supply different instances depending on circumstances, mock it out for testing etc. Otherwise the class would just create it internally and you'd have no control over it. This is particularly key for configurable components, components that talk across the network, heavyweight components etc.
In your specific example, dependency injection would look like:
public class MyParser {
private final JsonParser parser;
public MyParser(JsonParser parser) {
this.parser = parser;
}
public Result parse(JsonInput input) {
Result = parser.parse(input);
}
}
Assuming that JsonParser is an interface, you can now provide any implementation of that interface to MyParser which does not depend on which specific implementation you choose to use.
Let's say you have a class the daes the job called MyCustomParser .
Now you want to allow to inject the parser in that class to obtain a different behavior against the type of Parser you will inject (the json could change,you would like to use another way to parse etc).
So you should create an Interface - IParser with a method called (by instance) ParseIt
Now you can have different implementation of this method depending on how you want to do the parsing.
All you need to do is to pass this Interface to the class that will use it MyCustomParser (you might do that passing it as parameter in the constructor).
In this case you are injecting the Parser in the MyCustomParser class
Dependency injection, in the strict sense, has nothing to do with importing classes or adding jar files to your classpath. It is meant to support the programming to interfaces principle and it's a form of Inversion of Control.
The main benefit it brings is that your code does not depend on explicit dependency implementation, but it uses abstractions instead. You depend on an interface and some instance of an implementing class for that interface will be "injected" in your class at runtime (either as a constructor parameter, some setter, or -- bad choice, in my opinion -- instance field).
As a consequence, you can for example change your persistence layer without touching the code in your service components, because your service classes only care about some DAO/Repository interfaces and you'll provide them with different implementations under the hood.
Dependency injection is more about reducing the instances where a class has to create instances of things it depends upon for itself.
Dependency injection aims to instead 'give' those dependencies to the consuming class via some external mechanism. Common methods include passing the dependencies in via the constructor, or setting them into a publicly 'settable' property of the class.
For example, dependency injection via the constructor could look something like this:
public class Parser
{
private IJsonParser parser;
public Parser(IJsonParser parser) {
this.parser = parser;
}
}
Which removes the need for the Parser class to do something like this:
public class Parser
{
private IJsonParser parser;
public Parser() {
this.parser = new JsonParser();
}
}
This follows from the principles of single responsibility - the Parser class doesn't need to be responsible for creating a JsonParser, nor should it care what specific JsonParser it uses - its concern is simply that it requires something that does the work specified by the IJsonParser interface, and leave it up to some higher controlling code to determine the appropriate concrete type that is best suited to doing that work.
Related
This is a question that has been bothering me for a while.
In frameworks like Jersey we have interface(s) that we can subclass to add some functionality to our program. for example to add request filtering in a RESt application we can implement ContainerRequestFilter then Voila we got authentication.
My question is how does the framework/library know that we have subclass one of its interfaces?
as per my understanding you can't instantiate an interface, only its subclass such as:
public interface Greeter{
void sayHi();
}
public class SpanishGreeter implements Greeter{
#override
void sayHi(){
System.out.println("Hola");
}
}
public class Controller{
public void main(String[] args){
//We must know the name of subclass to instantiate?
Greeter spanishG = new SpanishGreeter();
}
}
What you're looking for is classpath scanning.
This isn't a trivial task, and it's pretty much what the name says: you basically need to scan the classpath and load every class you find (although the typical implementation will allow you to limit your search to certain packages and its subpackages only, to stop from things going really crazy).
There are no special tricks in the Java language that would make this task easy.
The good news is that you don't need to write it from scratch, as frameworks like Spring already have it built in, or if you want to have complete low-level control, you can use a dedicated library like Reflections.
I think instanceof keyword helps in it.
System.out.println(spanishG instanceof Greeter); // true
That happens due to Polymorphism. The code you mentioned
Greeter spanishG = new SpanishGreeter();
That has reference of parent(in you case interface). On run time it checks weather there is a child class implementing the method if yes then it calls child class behaviour. And in your class you are instantiating with child class on run time JVM knows you have provided implementation.
I want to achieve DYNAMIC dependency injection. Does GUICE support this? If not could you recommend any other DI framework?
The implementation that should be used for injection via #Inject must be determined during runtime e.g. via interaction with the user.
Similar to these questiones:
http://www.panz.in/2008/12/dynamic-dependency-injection.html
http://www.panz.in/2008/12/dynamic-dependency-injection.html
Thank you
The implementation needs to vary based on input, at some point you're going to have to resolve the input into some kind of class.
If you want that mapping to live in Guice, then you're basically getting an implementation based on a parameter, which maps to the SO question I just answered here. You can write a small injectable class that takes the input and returns a fully-injected implementation.
If you already have that mapping and have (for instance) a class literal in a variable, then you can just inject an Injector directly and ask it for the implementation.
class YourClass {
#Inject Injector injector;
SomeInterface yourMethod(String input) {
Class<? extends SomeInterface> clazz = getClassLiteralFromInput(input);
return injector.getInstance(clazz);
}
Class<? extends SomeInterface> getClassLiteralFromInput(String input) {
// Implement this as needed.
return SomeInstance.class;
}
}
Note that while you can always inject an Injector, you should only do so when you really don't know what kind of implementation you need (like here). In general you should inject the SomeInstance itself, or a Provider<SomeInstance> if you want to delay the creation.
We had similar requirement once, so what we did was use a factory pattern and add all the implementations in factory class implementation using spring.
That ways, when on run time we would know which implementation to use, we would make a call to my factory to provide implementation class.
Also, anytime you have more implementations you could update spring configuration for factory class.
This may not be close to design you have in mind, but this solved the purpose for us.
Cheers !!
What is the difference between creating a new object and dependency injection? Please explain in detail.
Well, they're not exactly comparable. You will always have to create a new object by instantiating a class at some point. Dependency injection also requires creating new objects.
Dependency injection really comes into play when you want to control or verify the behavior of instances used by a class that you use or want to test. (For Test Driven Development, dependency injection is key for all but the smallest example).
Assume a class Holder which requires an object of class Handle. The traditional way to do that would be to let the Holder instance create and own it:
class Holder {
private Handle myHandle = new Handle();
public void handleIt() {
handle.handleIt();
}
}
The Holder instance creates myHandle and no one outside the class can get at it. In some cases, unit-testing being one of them, this is a problem because it is not possible to test the Holder class without creating the Handle instance which in turn might depend on many other classes and instances. This makes testing unwieldy and cumbersome.
By injecting the Handle instance, for example in the constructor, someone from the outside becomes responsible for the creation of the instance.
class Holder {
private Handle myHandle;
public Holder(Handle injectedHandle) {
myHandle = injectedHandle;
}
public void handleIt() {
handle.handleIt();
}
}
As you can see the code is almost the same, and the Handle is still private, but the Holder class now has a much loser coupling to its outside world which makes many things simpler. And when testing the Holder class a mock or stub object can be injected instead of a real instance making it possible to verify or control the interaction between the Holder, its caller and the handle.
The actual injection would take place at some other place, usually some "main" program. There are multiple frameworks that can help you do that without programming, but essentially this is the code in the "main" program:
...
private Handle myHandle = new Handle(); // Create the instance to inject
private Handler theHandler = new Handler(myHandle); // Inject the handle
...
In essence, the injection is nothing more than a fancy set method. And of course, you can implement the injection mechanism using that instead of in the constructor like the simple example above.
Of course, both create objects. The difference is in who is responsible for the creation. Is it the class that needs its dependencies or a container like Spring for example, which wires the component's dependencies? You configure the dependencies in a separate(typically XML) configuration file.
It is really a separation of concerns. The class says I need this, this, and this component to function properly. The class doesn't care how it gets its components. You plug them into the class with a separate configuration file.
To give you an example let's consider having a shopping class that needs a payment module. You don't want to hardcode which payment module will be used. To achieve this you inverse the control. You can change the used payment module with a few keystrokes in the configuration file of the container. The power is that you aren't touching any Java code.
Well,
creating a new object is as explicit as it can get - you create a new instance of the desired class.
Dependency injections is a mechanism that provides you with references where you need them.
Imagine a class that represents a connection pool to your database - you usually only have one instance of that class. Now you need to distribute that reference to all the classes that use it.
Here is where Dependency Injection comes in handy - by using a DI framework such as Spring you can define that the one instance of your pool will be injected into the classes that need it.
Your question itself is not easy to answer since the creation of an object and dependency injection can't be compared that easily...
Dependency injections adds a layer of configurability into your application. In the sense, when you hard code object construction, you need to re-build and re-deploy your app, but when you use dependency injection, you can re configure the XML and change the behavior without re-building and re-deploying. There are a large variety of use cases where this can save a lot of tie and effort.
When using an inversion-of-control container to perform dependency injection, the container creates the objects, and not the developer. This is done so that the container can "inject" these objects into other objects.
The answer to the following question may also give the answer you are looking for: Why is the new operator an anti-pattern? Well, the simple answer is that using the new operator may create a hidden, inaccessible dependency within the containing class. This makes testing the containing class more difficult because it involves testing the hidden dependency at the same time (barring MOCK frameworks of course). However, you can avoid this situation by not using the new operator and injecting the dependent object instead. This also has the following advantages:
For test purposes you can inject a different object.
The resulting containing class is more reusable because it can support different implementations of the dependent object.
I'm writing (well, completing) an "extension" of Java which will help role programming.
I translate my code to Java code with javacc. My compilers add to every declared class some code. Here's an example to be clearer:
MyClass extends String implements ObjectWithRoles { //implements... is added
/*Added by me */
public setRole(...){...}
public ...
/*Ends of stuff added*/
...//myClass stuff
}
It adds Implements.. and the necessary methods to EVERY SINGLE CLASS you declare. Quite rough, isnt'it?
It will be better if I write my methods in one class and all class extends that.. but.. if class already extends another class (just like the example)?
I don't want to create a sort of wrapper that manage roles because i don't want that the programmer has to know much more than Java, few new reserved words and their use.
My idea was to extends java.lang.Object.. but you can't. (right?)
Other ideas?
I'm new here, but I follow this site so thank you for reading and all the answers you give! (I apologize for english, I'm italian)
If it is only like a "research" project in which you want to explore how such extension would work, you could provide your own implementation of the Object class. Simply copy the existing object implementation, add your setRole method etc, and give -Xbootclasspath:.:/usr/lib/jvm/java-6-sun/jre/lib/rt.jar as parameter to the java command. (I will look for api-classes in . before looking in the real rt.jar.)
You should consider using composition rather than inheritence to solve this problem; that way you can provide the functionality you need without using up your "one-shot" at inheritence.
For example, the JDK provides a class PropertyChangeSupport, which can be used to manage PropertyChangeListeners and the firing of PropertyChangeEvents. In situations where you wish to write a class that fires PropertyChangeEvents you could embed a PropertyChangeSupport instance variable and delegate all method calls to that. This avoids the need for inheritence and means you can supplement an existing class hierarchy with new functionality.
public class MyClass extends MySuperClass {
private final PropertyChangeSupport support;
public MyClass() {
this.support = new PropertyChangeSupport(this);
}
public void addPropertyChangeListener(PropertyChangeListener l) {
support.addPropertyChangeListener(l);
}
protected void firePropertyChangeEvent() {
PropertyChangeEvent evt = new ...
support.firePropertyChangeEvent(evt);
}
}
you can extend Object - every class extends it.
you seem to need something like multiple inheritance - there isn't such a thing in Java
if you want to add functionality, use object composition. I.e.,
YourClass extends Whatever implements ObjectWithRoles {
private RoleHandler roleHandler;
public RoleHandler getRoleHandler() {..} // defined by the interface
}
And then all of the methods are placed in the RoleHandler
If you're talking about adding a role to all your objects I would also consider an annotation-based solution. You'd annotate your classes with something like #Role("User"). In another class you can extract that role value and use it.
I think it would need an annotation with runtime retention and you can check, run-time, whether the annotation is present using reflection and get that annotation using getAnnotation. I feel that this would be a lot cleaner than extending all your classes automatically.
I believe there are some frameworks which use exactly such a solution, so there should be example code somewhere.
If you are doing what you are doing, then inheritance is probably not the correct idiom. You may want to consider the decorator pattern, whereby you construct a class that takes as its parameter some other class with less functionality, and adds some additional functionality to it, delegating to the existing class for functionality that already exists. If the implementation is common to many of your decorators, you may want to consider putting that functionality in class that can be shared and to which you can delegate for all your decorators. Depending on what you need, double-dispatch or reflection may be appropriate in order to make similar but not quite the same decorators for a large variety of classes.
Also, as has been pointed out in the comments, String is declared "final" and, therefore, cannot be extended. So, you should really consider a solution whereby you delegate/decorate objects. For example, you might have some object that wraps a string and provides access to the string via getString() or toString(), but then adds the additional functionality on top of the String class.
If you just want to associate some objects with additional attributes, use a Map (e.g. HashMap).
What you really want to do would be monkey patching, i.e. changing the behaviour of existing classes without modifying their code.
Unfortunately, Java does not support this, nor things like mixins that might be used alternatively. So unless you're willing to switch to a more dynamic language like Groovy, you'll have to live with less elegant solutions like composition.
I'm working with some existing code, trying to add to it and increase the unit tests for it. But running into some problems with getting the code testable.
Original Constructor:
public Info() throws Exception
{
_ServiceProperties = new ServiceProperties();
_SshProperties = new SshProperties();
}
I'm aware that this is bad, and obviously not testable. In a junit environment, this class will fail to create every time since it wont be able to find the necessary properties to construct itself. Now, I'm aware this class would be a lot more testable with the simple change of moving anything prefaced with 'new' as a parameter.
So I end up with:
New Constructor:
public Info(ServiceProperties srvProps, SshProperties sshProps) throws Exception
{
_ServiceProperties = srvProps;
_SshProperties = sshProps;
}
Which allows me to properly unit test this Info class. The problem though, is now all that work is pushed to some other class:
Some Other Class' Method:
public void useInfo() throws Exception
{
ServiceProperties srvProps = new ServiceProperties();
SshProperties sshProps = new SshProperties();
Info info = new Info(srvProprs, sshProprs);
doStuffWithInfo(info);
}
Now this method isn't testable. All I've managed to do is push off where the constructions of these Property objects are occurring, and somewhere else some piece of code is going to be stuck actually having to call "new".
Here's the rub for me: I can't figure out how to break this chain of events of simply pushing these "new" calls somewhere else. What am I missing?
Look at using a Dependency Injection framework such as Spring. This application of Inversion of Control means that each of your types can avoid the pitfall you've seen, leaving the configuration to "wire" components together.
This introduction to Spring (pdf) gives a comprehensive overview of Spring. The first chapter or two should be sufficient to explain the concepts.
Also see Inversion of Control Containers and the Dependency Injection pattern by Martin Fowler
You have the right idea. Perhaps this will help you. I recommend you follow two rules for all your classes of significance, where "of significance" means if you don't follow the steps it will be more difficult to test, reuse, or maintain the class. Here are the rules:
never instantiate or self-acquire a dependency
always program to interfaces
You have a start at rule #1. You changed your Info class to no longer create its dependencies. By "dependency" I mean other classes, configuration data loaded from property files or whatever, etc. When you depend on how something is instantiated you are tying your class to it and making it more difficult to test, reuse and maintain. So, even if a dependency is created via a factory or a singleton, don't have your class create it. Have something else call create() or getInstance() or whatever and pass it in.
So you chose the "something else" to be the class that uses your class, and realized there is a bad smell to it. The remedy is to instead have the entry-point to your application instantiate all dependencies. In a traditional java app, this is your main() method. if you think about it, instantiating classes and hooking them up to each other, or "wiring" them together, is a special kind of logic: "application assembly" logic. Is it better to spread this logic throughout your code, or to collect it in one place to more easily maintain it? The answer is that collecting it in one place is better - not only for maintainance, but the act of doing so turns all your classes of significance into more useful and flexible components.
In your main() or equivalent of main() you should create all the objects you need, passing them into each others' setters and constructors to "wire" them together. Your unit tests would then wire them differently, passing in mock objects or similar things. The act of doing all this is called "dependency injection". After doing as I say, you will likely have a big ugly main() method. This is where a dependency injection tool can help you out and in fact make your code infinitely more flexible. The tool I would suggest when you get to this point, as others have also suggested, is Spring.
The less important rule #2 - always program to interfaces, is still very important because it eliminates all dependencies on implementation, making reuse much easier, not to mention leveraging other tools like mock object frameworks, ORM frameworks, etc. easier as well.
Even dependency injection frameworks like Spring, Guice, PicoContainer etc. need some sort of boostrap so you always have to build something up.
I would suggest you to use a provider/factory that returns a configured instance of you class. This would allow you to exit the "creation"-hierarchy.
Your constructors aren't incorrect and the problem isn't about when/where code is executed, it's about what everyone else mentioned: Dependency Injection. You need to create mock SshProperties objects to instantiate your object. The simplest way (assuming the class isn't marked as final) is to extend the class:
public class MockSshProperties extends SshProperties {
// implemented methods
}
You can you use mock frameworks like Mockito:
public class Info {
private final sshProps;
private final serviceProps;
public Info() {
this(new SshProperties(), new ServiceProperties());
}
public Info(SshProperties arg1, ServiceProperties arg2) {
this.sshProps = arg1;
this.serviceProps = arg2
}
}
public class InfoTester
{
private final static SshProperties sshProps = mock(SshProperties.class);
private final static ServiceProperties serviceProps = mock(ServiceProperties.class);
static {
when(sshProps.someGetMethod("something")).thenReturn("value");
}
public static void main(String[] args) {
Info info = new Info(sshProps, serviceProps);
//do stuff
}
}
The easiest answer is Spring. However another answer is to put your config stuff into JNDI.
Spring in some ways is a better answer, especially if you don't have anything that changes depending on environment.
You let the some-other-class have too much knowledge about the Info class and its dependencies. A dependency injection framework would use a provider class. Using generic types one can make a Provider of Info objects:
interface Provider<T> {
T get();
}
If your some-other-class take a Provider<Info> in its constructor your method would look like:
public void useInfo() throws Exception
{
Info info = infoProvider.get();
doStuffWithInfo(info);
}
This has removed construction of concrete classes from your code. Next step is to make Info into an interface to make it easier to create a mock for the specific unit-test case.
And yes, this will push and push all the object construction code further and further up. It will lead to some module that only describes how to wire things together. The "rules" for such code is that it should be free of conditional statements and loops.
I recommend reading Misko Heverys blog. All the presentations are useful and printing out the guide to writing testable code as little rulebook once you understand the rules is a good thing.