tags: Spring Dependency Injection
Autowiring feature of spring framework enables you to inject the object dependency implicitly. It internally uses setter or constructor injection.
Advantages:
- It requires the less code because we don’t need to write the code to inject the dependency explicitly. Disadvantages:
- No control of programmer.
- It can’t be used for primitive and string values.
Autowiring Modes
| No. | Mode | Description |
|---|---|---|
| 1) | no | It is the default autowiring mode. It means no autowiring bydefault. |
| 2) | byName | The byName mode injects the object dependency according to name of the bean. In such case, property name and bean name must be same. It internally calls setter method. |
| 3) | byType | The byType mode injects the object dependency according to type. So property name and bean name can be different. It internally calls setter method. |
| 4) | constructor | The constructor mode injects the dependency by calling the constructor of the class. It calls the constructor having large number of parameters. |
| 5) | autodetect | It is deprecated since Spring 3. |
1) byName autowiring mode
In case of byName autowiring mode, bean id and reference name must be same. It internally uses setter injection.
<bean id="b" class="org.sssit.B"></bean>
<bean id="a" class="org.sssit.A" autowire="byName"></bean> But, if you change the name of bean, it will not inject the dependency. Let’s see the code where we are changing the name of the bean from b to b1.
<bean id="b1" class="org.sssit.B"></bean>
<bean id="a" class="org.sssit.A" autowire="byName"></bean>2) byType autowiring mode
In case of byType autowiring mode, bean id and reference name may be different. But there must be only one bean of a type.
It internally uses setter injection.
<bean id="b1" class="org.sssit.B"></bean>
<bean id="a" class="org.sssit.A" autowire="byType"></bean> In this case, it works fine because you have created an instance of B type. It doesn’t matter that you have different bean name than reference name.
But, if you have multiple bean of one type, it will not work and throw exception.
<bean id="b1" class="org.sssit.B"></bean>
<bean id="b2" class="org.sssit.B"></bean>
<bean id="a" class="org.sssit.A" autowire="byName"></bean> 3) constructor autowiring mode
In case of constructor autowiring mode, spring container injects the dependency by highest parameterized constructor.
If you have 3 constructors in a class, zero-arg, one-arg and two-arg then injection will be performed by calling the two-arg constructor.
<bean id="b" class="org.sssit.B"></bean>
<bean id="a" class="org.sssit.A" autowire="constructor"></bean>4) no autowiring mode
In case of no autowiring mode, spring container doesn’t inject the dependency by autowiring.
<bean id="b" class="org.sssit.B"></bean>
<bean id="a" class="org.sssit.A" autowire="no"></bean>