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Hunor Vadasz-Perhat
Hunor Vadasz-Perhat

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spring-005: why-spring-cannot-match-by-name-for-injection

Why Spring Cannot Automatically Match by Name

  1. Spring’s Injection Mechanism is Type-First

    • Spring performs dependency injection primarily by type, not by name.
    • When it encounters multiple beans of the same type (TenantDataSource), it doesn't automatically fall back to matching by parameter name.
    • Matching by parameter name is not a default behavior because Spring cannot always guarantee that parameter names in your method will match bean names.
  2. Parameter Names Are Not Always Retained

    • In Java, parameter names are not retained in the compiled bytecode by default. Without special configuration, the parameter names (tenantADataSource and tenantBDataSource) are erased during compilation and replaced with generic names like arg0 and arg1.
    • Without parameter names, Spring has no way of knowing that tenantADataSource refers to the bean named "tenantA-dataSource".

### Solution for Parameter Name Retention:
You can use the -parameters compiler flag to retain parameter names in the bytecode:

   javac -parameters MyClass.java
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Even with this flag, Spring still does not match by parameter name unless explicitly configured.

  1. Ambiguity is a Design Safeguard
    • Spring avoids making assumptions to prevent unintended behavior:
      • What if the parameter names were accidentally swapped or incorrectly named?
      • What if the developer expected a different mapping?

For example:

   @Autowired
   public TenantService(TenantDataSource tenantBDataSource, TenantDataSource tenantADataSource) {
       this.tenantADataSource = tenantADataSource; // Seems backward
       this.tenantBDataSource = tenantBDataSource; // Seems backward
   }
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  • Without explicit guidance (e.g., @Qualifier), Spring cannot determine the developer's intent and chooses to throw an error instead of potentially injecting the wrong beans.
  1. Explicit Is Better Than Implicit

    • Spring follows the "explicit is better than implicit" principle. Dependency injection should be predictable and not rely on assumptions like parameter-name-to-bean-name matching.
  2. Framework Neutrality

    • Spring operates in a way that is neutral across different environments and frameworks. Some languages or frameworks (like Kotlin) retain parameter names by default, but others don’t, so Spring avoids depending on parameter names for critical functionality.

When Name Matching Can Work Automatically

While Spring doesn't match by parameter name by default, it can match by parameter name in some cases, but it depends on your setup:

Spring 4.3+ Parameter Name Discovery

  • If you compile your code with the -parameters flag, Spring can infer bean names from parameter names when there’s ambiguity.

Example:

@Autowired
public TenantService(TenantDataSource tenantADataSource, TenantDataSource tenantBDataSource) {
    this.tenantADataSource = tenantADataSource;
    this.tenantBDataSource = tenantBDataSource;
}
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  • If the -parameters flag is used and the parameter names (tenantADataSource, tenantBDataSource) match the bean names, Spring can resolve the ambiguity.
  • However, this behavior is not always reliable or predictable in complex configurations, which is why explicit configuration (e.g., @Qualifier) is preferred.

Why Spring Prefers @Qualifier

Using @Qualifier ensures clarity and eliminates ambiguity, making the intent explicit to both the developer and the framework.

Example:

@Autowired
public TenantService(
    @Qualifier("tenantA-dataSource") TenantDataSource tenantADataSource,
    @Qualifier("tenantB-dataSource") TenantDataSource tenantBDataSource
) {
    this.tenantADataSource = tenantADataSource;
    this.tenantBDataSource = tenantBDataSource;
}
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  • This way, Spring knows exactly which beans to inject, regardless of parameter names or other factors.

Should Spring Be Smarter?

One might argue that frameworks like Spring should be smarter and automatically match bean names to parameter names when there’s ambiguity. However:

  1. Backward Compatibility:

    • Automatically changing this behavior could break existing applications where bean names and parameter names do not match.
  2. Predictability:

    • Explicitly resolving ambiguity (e.g., via @Qualifier) ensures developers always know which beans are being injected.
    • Implicit assumptions can lead to hard-to-debug issues, especially in large, complex applications.

What About Real-Life Scenarios?

In most real-life applications:

  • Developers use meaningful names for beans and parameters, but they also prefer explicit configuration like @Qualifier to ensure clarity.
  • For multi-instance scenarios (like your example), dynamic configurations or factory-based approaches (e.g., AbstractRoutingDataSource) are often used instead of manually wiring individual beans.

Key Takeaways

  1. Spring Injects by Type First:

    • If there’s only one bean of a type, Spring injects it without issues.
    • If multiple beans of the same type exist, Spring needs additional hints (e.g., @Qualifier) to resolve ambiguity.
  2. Parameter Names Are Not Reliable:

    • By default, parameter names are not retained in Java bytecode, so Spring cannot use them to match beans to parameters.
    • Even with parameter name retention (-parameters), Spring avoids implicit name-based matching unless explicitly configured.
  3. Explicit Over Implicit:

    • Spring prioritizes predictability and encourages developers to use explicit configurations (e.g., @Qualifier or @Primary) over relying on implicit matching.
  4. Spring’s Design Philosophy:

    • Ambiguity errors prevent unintended behavior and ensure developers have full control over dependency injection.

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