I followed the following example of dependency injection: http://www.tutorialspoint.com/spring/spring_autowired_annotation.htm
For example the TextEditor class (from the above link):
public class TextEditor {
private SpellChecker spellChecker;
#Autowired
public void setSpellChecker( SpellChecker spellChecker ){
this.spellChecker = spellChecker;
}
public SpellChecker getSpellChecker( ) {
return spellChecker;
}
public void spellCheck() {
spellChecker.checkSpelling();
}
}
How can these dependencies/classes be instantiated, while they don't have any constructor?
Is Java simply making an object of that type, that is empty? Like an empty parameter constructor without any code?
Thanks for making this more clear!
Unless specified otherwise, every Java class has the default constructor. So here, you have a default public TextEditor() constructor, even though you haven't coded for it. (You could code it if you needed to change its visibility from public, declare a thrown exception, etc.)
So yes, Spring calls this default constructor - then calls the setSpellChecker method (as annotated, and through reflection) to populate it.
If no constructor is defined, a class can be instantiated via the no-argument default constructor.
So, the framework calls that constructor (supposedly using reflection) and then uses the set method to set the one field of the freshly created class.
The example above is using Spring annotations and Spring context file and those are the main and most important parts of the project, considering the DI.
So in the context file you have following line:
<!-- Definition for spellChecker bean -->
<bean id="spellChecker" class="com.tutorialspoint.SpellChecker">
</bean>
this defines a class with reference spellChecker that mapps to a class com.tutorialspoint.SpellChecker and once the compiler find such property in a method that is marked as #Autowired on instantiation of the object it injects/sets the relevant version of the required dependency.
In cases where a property doesn't match a reference tag in the applicationContext.xml file Spring is trying to map the type e.g. property with name mySpecialSpellChecker which has type of com.tutorialspoint.SpellChecker still will be mapped to bean with id="spellChecker" if there are more than one of same type Spring won't instantiate your object and you might get compile time error as Spring can't know which version of the two (or more) is the correct one so this requires developer input.
This is the order of execution:
instantiate textEditor, this has default constructor that is not visible in the code public TextEditor ()
the new instance is set in a pool of available objects with reference textEditor
instantiate spellChecker and add to the pool of available object with relevant reference/label
all #Autowired properties/methods are set/called with relevant objects in this case Spring calls: setSpellChecker(spellChecker)
Related
Basically, I want to make this code work:
#Component
abstract class MyBaseClass(val myArg: MyArgClass) {
#Autowired
private lateinit var myInjectedBean: MyInjectedBean
fun useBothArgAndBean()
}
class MyConcreteClass(myArg: MyArgClass) : MyBaseClass(myArg)
val myConcreteClass = MyConcreteClass(obtainMyArgClass())
myConcreteClass.useBothArgAndBean()
So I have a base class with one argument in the constructor and another argument injected from the Spring context. Currently, such a setup is not possible because Spring tries to inject also MyArgClass from context and because there's no such bean (it's constructed manually) it fails on "no matching bean".
The question is how to make this scenario work. Note that I cannot use the factory-method solution mentioned here https://stackoverflow.com/a/58790893/13015027 because I need to directly call MyBaseClass constructor from MyConcreteClass constructor. Perhaps there's a trick on how to avoid that or how to force Spring not to try to inject into the base constructor or ...?
You have a quite confusing setup there, and I am not sure that you are fully aware how injection by Spring works. You can
either create a class on your own, using its constructor, or
you can let Spring create the class and inject everything, and you don't call the constructor.
When you call the constructor, Spring will not magically inject some parts of your class, just because it has seemingly the right annotations. The variable myInjectedBean will just be null.
If you want to have the ability to create the class on your own using the constructor, you should not use field injection, because you would obviously not have any possibility to initialize the field.
Then your classes MyBaseClass and MyConcreteClass would look like this:
abstract class MyBaseClass(
val myArg: MyArgClass,
private val myInjectedBean: MyInjectedBean
) {
fun useBothArgAndBean()
}
class MyConcreteClass(myArg: MyArgClass, myInjectedBean: MyInjectedBean) : MyBaseClass(myArg, myInjectedBean)
Now, as already suggested by #Sam, you can have myInjectedBean be injected while providing myArg manually by writing another component that can completely be created by Spring, because it will only autowire myInjectedBean while myArg is provided as argument for a factory method:
#Component
class MyFactory(val myInjectedBean: MyInjectedBean) {
fun createMyConcreteClass(myArg: MyArgClass) =
MyConcreteClass(myArg, myInjectedBean)
}
Then in a class, where you have an injected myFactory: MyFactory you can just call myFactory.createMyConcreteClass(myArg) and you obtain a new MyConcreteClass that has an injected myInjectedBean.
I think you still do need some sort of factory. It would pass both the bean and the additional arguments to the MyConcreteClass constructor, and would look like this:
#Component
class MyFactory(val myInjectedBean: MyInjectedBean) {
fun getMyConcreteClass(myArg: MyArgClass) =
MyConcreteClass(myArg, myInjectedBean)
}
If using that approach, none of the other classes except MyInjectedBean would need to be registered with Spring.
In fact, it's a little surprising to me that you currently have MyBaseClass annotated with #Component. What do you expect that to do, and does it work?
Why my code is failing with the below error when I AUTOWIRE the no arg constructor but it runs fine when I autowire only the single arg constructor
Exception in thread "main" java.lang.NullPointerException
Here is TennisCoach Class code snippet
#Component // this is bean id
public class TennisCoach implements Coach {
public TennisCoach(FortuneService thefortuneservice) {
System.out.println(" inside 1 arg constructter");
fortuneservice = thefortuneservice;
}
#Autowired
public TennisCoach() {
System.out.println(" inside 0 arg constructter");
}
}
Coach theCoach = myapp.getBean("tennisCoach", Coach.class);
System.out.println(theCoach.getDailyFortunes());
How are things working behind the scene?
If you have only one constructor in a class, you don't need to use #Autowired--Spring understands that there's only one option, so it uses that one. But you have two constructors, so you need to tell Spring which one to use.
When you invoke the default constructor, however, what do you expect to happen? You don't set up your variables anywhere at all, but you are trying to read from them.
Constructor with no arguments doesn't want you put #Autowired on it as you are not injecting anything in its arguments (since there are none). #Autowired is required for constructor with arguments however.
#Component annotation above the class will create your bean via calling the default constructor and you can use that anywhere but need to make sure that you don't call anything on the fortuneservice as that will result in a null ptr exception unless you initialize it. On the contrary, if you put #Autowired annotation on top of constructor with argument and it runs fine then that means that the fortuneservice bean that you have declared elsewhere will now be injected inside this class and no null ptr exception if you call some method on that.
Adding to the previous to answers - think about the term "constructor injection". You are INJECTING references via constructor parameters. When you #Autowire an empty constructor, there isn't anything to inject. Therefore you get a NullPointerException when attempting to access the field that was supposed to have a reference injected into.
Given the following Java code (from a 3rd-party-library outside of my control):
package some.third.party.lib;
interface MyInterface { ... }
and the following class A with a package-private constructor (this is its only constructor):
package some.third.party.lib;
[...]
class A implements MyInterface {
A() {}
}
ProxyFactoryBean.setTargetName expects the ID of an already instantiated bean.
Since my Spring Java Config class is in an application-specific package (and I don't want to change it to some.third.party.lib), I can't instantiate class A, since it's package-private.
I'm aware that I could use reflection to temporarily change the visibility of A's constructor, but I would like to avoid this if possible.
Q: (How) can I create a Spring (4.2.1.RELEASE) ProxyFactoryBean of a class A in Spring Java Config without having to use reflection (and without having to put my Java Config Class into the same package as A)?
Please note that when using Spring XML config, this situation does not occur, because Spring (in the background) creates a Bean for class A, probably also using reflection.
I having a Spring class, where I am autowiring a service using constructor, plus in the same constructor I am intializing other field of the same class.
#Component
class Converter {
private TestService testService;
private Interger otherFields;
#Autowired
public Converter(TestService testService) {
this.testService = testService;
this.otherFields = new Integer(10);
}
}
My Functionality is working fine, but is it a good practice?, would #Autowired annotation have any impact on otherFields intialization process
It shouldn't. Back in the xml days, when you want to pass on an argument to a constructor, you mentioned your ref bean for the constructor arg. This just means that you must have a constructor that takes the specified bean type as an argument. It doesn't really matter what you add in the constructor, as long as you are creating a valid object through the constructor (though this is just normal java programming and nothing to do with Spring).
Auto-wiring is just an easy way to create your object with the necessary dependencies and your code is still your code.
No.
When Spring is instantiating your class it will locate the constructor annotated with #Autowired, collect the beans that corresponds to the arguments the constructor takes and then invoke it with those beans as arguments.
It will then scan through all fields and methods in your class and inject beans into any fields that are annotate with #Autowired. It will not touch methods or fields that are not annotated.
I have a web application that executes on tomcat 6.
I have a MysqlDb class that uses a BasicDataSource from a spring JDBC.
so far I've used the following bean configuration in web.xml:
<bean id="MysqlDb" class="com.xpogames.gamesisland.mysql.MysqlDb">
<property name="idDataSource" ref="idDataSource"/>
</bean>
and I had the following setter function:
public void setidDataSource(BasicDataSource ds) {
this._dataSource=(DataSource)ds;
this._simpleJdbcTemplate = new SimpleJdbcTemplate(_dataSource);
this._jdbcTemplate = new JdbcTemplate(_dataSource);
}
I want to convert my class to use static functions, so I created an empty private constructor so the class won't explicitly instantiated by callers.
besides that I changed the setidDataSource function to a static function, but when I try to do that I get the following error:
Error setting property values; nested exception is org.springframework.beans.NotWritablePropertyException: Invalid property 'idDataSource' of bean class [com.xpogames.gamesisland.mysql.MysqlDb]: Bean property 'idDataSource' is not writable or has an invalid setter method. Does the parameter type of the setter match the return type of the getter?
is there a way to resolve this issue in web.xml or do I need to manually
fetch the ServletContext
ServletContext servletContext = this.getServletContext();
this._context = WebApplicationContextUtils.getRequiredWebApplicationContext(servletContext);
and fetch the bean from there and just remove the lines i printed here from web.xml ?
For one, you've declared the setter setidDataSource. It should be setIdDataSource. The first letter of a property must be a capital letter after the word set.
Also, a setter method must not be static but an instance method.
Spring-beans are by default singletons, you do not need to implement your class as a singleton as long as you use the bean from the context.
The simplest answer to your question is getting the bean from the context after setting the datasource in the context, but I thought you want to stay away from the context.
Static setters for class are of course possible (they would just set static property for all instances), but in IoC pattern (which is used in spring), bean is instance, and term "property of bean" always means "property of instance of class" - consider it as limitation of given IoC implementation.
I understand know that I had a bad implementation idea. I still need a constructor, so building a class with static functions and static init is a bad idea, and trying to execute a static setter from a bean is not logical and impossible.
Instead I changed the class to be a singleton class, so I will be able to use it anywhere in my application and it will be constructed only once.
thanks for all the information.
update
I still don't know if that's a good method, but at least it works.
in my red-web.xml (consider it as spring applicationContext.xml), I have the following:
<bean id="MysqlDb" class="com.xpogames.gamesisland.mysql.MysqlDb" init-method="getInstance">
<property name="idDataSource" ref="idDataSource"/>
</bean>
Here it creates a MysqlDb bean and configure it to use the getInstance() init method if MysqlDb Class. i made sure to have a setidDataSource() function in mysqlDb class for the datasource to be properly set.
<bean id="web.handler" class="com.xpogames.gamesisland.Application">
<property name="MysqlDb" ref="MysqlDb"/>
</bean>
Here, I create the main bean of my application and I made sure to have the function setMysqlDb for the MysqlDb class to be set from the bean configuration.
So far mysqlDb acts as a singelton class because it's constructor is protected and it creates the instance only once:
public static MysqlDb getInstance() {
if (instance == null) {
instance = new MysqlDb();
}
return instance;
}
The problem that I encountered was that in other parts of my application whenever I used getInstance(), the MysqlDb class would come up and all the variables that where set with setidDataSource where null.
so resolve that issue I created another function called setInstance in mysqlDb:
public static void setInstance(MysqlDb db) {
instance=db;
}
this is my main setMysqlDb function in my main application:
public void setMysqlDb(MysqlDb db) {
this._mysqlDb=db;
/* it seems that without forcing a setInstance on the class, whenever other classes
* would try to getInstance(), variables that are supposed to be configured by the bean
* would be empty. this resolves that issue
*/
MysqlDb.setInstance(db);
}
so this configuration works. it's obviously not the recommended or best solution! but it seems that I need to read and learn further before I come up with a better solution.