Introduce support for creating a MethodParameter from a Java 8 Parameter [SPR-14055]

[spring-projects-issues]

Sam Brannen opened SPR-14055 and commented

Status Quo

A MethodParameter or SynthesizingMethodParameter can be created from a Method and an index; however, there is currently no first-class support for creating a MethodParameter from a java.lang.reflect.Parameter.

Related Resources

The MethodParameterFactory in the spring-test project has factory methods for creating a MethodParameter or SynthesizingMethodParameter from a Java 8 Parameter.

The trick involves retrieving the java.lang.reflect.Executable from the supplied Parameter and then determining the index of the corresponding parameter.

The following code excerpts demonstrate the technique.

public static MethodParameter createMethodParameter(Parameter parameter) {
	Assert.notNull(parameter, "Parameter must not be null");
	Executable executable = parameter.getDeclaringExecutable();
	if (executable instanceof Method) {
		return new MethodParameter((Method) executable, getIndex(parameter));
	}
	// else
	return new MethodParameter((Constructor<?>) executable, getIndex(parameter));
}

private static int getIndex(Parameter parameter) {
	Assert.notNull(parameter, "Parameter must not be null");
	Executable executable = parameter.getDeclaringExecutable();
	Parameter[] parameters = executable.getParameters();
	for (int i = 0; i < parameters.length; i++) {
		if (parameters[i] == parameter) {
			return i;
		}
	}
	throw new IllegalStateException(String.format("Failed to resolve index of parameter [%s] in executable [%s]",
		parameter, executable.toGenericString()));
}

Deliverables

1. Support creation of a MethodParameter from a java.lang.reflect.Parameter.
2. Support creation of a SynthesizingMethodParameter from a java.lang.reflect.Parameter.

---

Issue Links:

• Introduce support for JUnit 5 in the TestContext framework [SPR-13575] #18151 Introduce support for JUnit 5 in the TestContext framework ("is depended on by")
• Support creation of SynthesizingMethodParameter for constructor parameter [SPR-14054] #18626 Support creation of SynthesizingMethodParameter for constructor parameter
• MethodParameter should not be equal to SynthesizingMethodParameter [SPR-14438] #19009 MethodParameter should not be equal to SynthesizingMethodParameter
• Validate method parameter index via Java 8 Method.getParameterCount() [SPR-13456] #18036 Validate method parameter index via Java 8 Method.getParameterCount()

Referenced from: commits 7e783dd, 5f53a60, 39e3f2e

[spring-projects-issues]

Sam Brannen commented

This issue is related to #18626.

[spring-projects-issues]

Juergen Hoeller commented

I'm leaning towards only doing this in 5.0 but then rearchitecting MethodParameter on java.lang.reflect.Parameter to begin with: internally just using Parameter but allowing construction from a given Method / Constructor and an index (which then just leads to immediate Parameter lookup for all subsequent purposes).

This would also allow us to simply introduce overloaded MethodParameter / SynthesizingMethodParameter constructors which accept a Parameter argument, with no need for a separate factory class. For that reason in particular, I'd rather only do it in 5.0.

Juergen

[spring-projects-issues]

Sam Brannen commented

Sounds good, Juergen.

I'd also like to avoid the introduction for a factory for this. I only introduced the factory in spring-test-junit5 in order to have better separation of concerns.

In any case, spring-test-junit5 has an interim solution for this until it's officially supported in Spring 5.

Cheers,

Sam

[spring-projects-issues]

Juergen Hoeller commented

I ended up adapting from Parameter since that class is no more sophisticated in its metadata caching than our MethodParameter is already. Also, we couldn't point to a return type (parameter index -1) in a uniform way then.

So as of 5.0, MethodParameter and SynthesizingMethodParameter have corresponding forParameter(Parameter)) factory methods, as well as forExecutable(Executable, parameterIndex)) as a replacement for the now-deprecated forMethodOrConstructor.