#10 Constructor and Setter Injection in Spring

Main ideas / lessons

• Goal: Explain how Spring (without Spring Boot) creates objects using an IoC container and how to use dependency injection to wire one object into another.
• Object creation in Spring Framework (XML config):
  • Use an XML file (e.g., spring.xml) with multiple <bean> definitions.
  • When the app runs, Spring reads these bean definitions and instantiates the objects inside the Spring container.
• Why injection is needed:
  • If a class only has a reference field (e.g., laptop), and Spring hasn’t injected it, the field remains null, leading to a NullPointerException when methods are called on it.
• Two types of injection taught:
  • Setter Injection (set values after object creation)
  • Constructor Injection (provide values during object creation)

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Detailed methodology / instruction list (as presented)

1) Configure bean creation (no Spring Boot)

1. Create an XML configuration file such as spring.xml.
2. Define beans using <bean> tags, following an example pattern like:
   • A bean named Dev of class Dev
   • A bean named Dev one of class dev/Dev (differentiated bean id)
   • A bean named laptop of class laptop
3. Result:
   • On startup, Spring creates these objects and stores them in the IoC / Spring container.
   • The container holds distinct instances per bean id (e.g., Dev, Dev one, laptop).

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2) Setter Injection for a primitive value (e.g., age)

• In the target class (e.g., Dev), include a field such as age (an int).
• Ensure accessors exist:
  • Create a getter and setter for the field (getAge() / setAge(...)).
• In spring.xml, inside the relevant <bean> definition for the correct dev bean (e.g., Dev):
  • Add a <property> entry, specifying:
    • name="age" (the field to set)
    • value="12" (the value Spring should inject)
• Verify behavior:
  • If Dev one does not define the age property, age remains default (0 for int).
  • If you remove/restore the property, the printed value changes accordingly.

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3) Constructor Injection for a primitive value (e.g., age)

• Modify the class to include a parameterized constructor, e.g.:
  • Dev(int age) that sets the internal age field.
• In spring.xml, switch injection style for the relevant bean (e.g., Dev one):
  • Use <constructor-arg> instead of <property>.
  • Provide:
    • value="14" (the constructor argument for the age parameter)
• Notes given:
  • If there is only one constructor parameter, you can omit specifying name.
  • If there are multiple parameters, you may specify:
    • name="..." (by parameter name), or
    • index="0", index="1" to map values in order.

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4) Injecting a reference type (e.g., laptop) into another bean

Problem demonstrated

• If Dev contains a laptop reference field but it isn’t injected, it stays null.
• Calling laptop.compile() then triggers a NullPointerException.

Setter injection for a reference

• In Dev, ensure you have getters/setters for the reference field (e.g., setLaptop(...)).
• In spring.xml, inside the target <bean> for Dev:
  • Add:
    • <property name="laptop" ref="labOne">
  • Use ref, not value.
• The ref must point to the bean id of the laptop bean in the container.
• Example adjustment shown:
  • Rename the laptop bean id (e.g., from laptop to lap one / lab one) so the reference matches.
• Additional requirement mentioned:
  • When using setter injection for laptop, Spring expects the field to be writable (has a setter).
  • If not, Spring can throw an error such as “not writable / invalid”.

Constructor injection for a reference

• Create a constructor in Dev that accepts Laptop (or the laptop type), e.g.:
  • Dev(Laptop laptop)
• In spring.xml, use:
  • <constructor-arg ref="labOne" /> for the constructor argument.

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Key concept comparison (stated)

• Use Constructor injection when the dependency is compulsory (must exist at object creation time).
• Use Setter injection when the dependency is optional (can be set later after the object is created).

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Speakers / sources

• Speaker: “welcome back aliens” (the video presenter / narrator; no name given in subtitles)
• Source: Spring Framework concepts demonstrated via the video’s own example classes/configuration (e.g., Dev, laptop, spring.xml)—no external named sources referenced.