Introduce ReflectiveIndexAccessor convenience class in SpEL

[sbrannen]

Overview

Somewhat analogous to the ReflectivePropertyAccessor implementation of PropertyAccessor, we should introduce a general purpose ReflectiveIndexAccessor implementation as a convenience for users.

However, ReflectiveIndexAccessor should implement CompilableIndexAccessor (instead of just IndexAccessor) in order to provide built-in compilation support.

A proof of concept has already been implemented in the tests for CompilableIndexAccessor:

spring-framework/spring-expression/src/test/java/org/springframework/expression/spel/SpelCompilationCoverageTests.java

Lines 7238 to 7333 in 27d2200

	/**
	* {@link CompilableIndexAccessor} that uses reflection to invoke the
	* configured read-method for index access operations.
	*/
	static class ReflectiveIndexAccessor implements CompilableIndexAccessor {
	private final Class<?> targetType;
	private final Class<?> indexType;
	private final Method readMethod;
	private final Method readMethodToInvoke;
	private final String targetTypeDesc;
	private final String methodDescr;
	public ReflectiveIndexAccessor(Class<?> targetType, Class<?> indexType, String readMethodName) {
	this.targetType = targetType;
	this.indexType = indexType;
	this.readMethod = ReflectionUtils.findMethod(targetType, readMethodName, indexType);
	Assert.notNull(this.readMethod, () -> "Failed to find method '%s(%s)' in class '%s'."
	.formatted(readMethodName, indexType.getTypeName(), targetType.getTypeName()));
	this.readMethodToInvoke = ClassUtils.getInterfaceMethodIfPossible(this.readMethod, targetType);
	this.targetTypeDesc = CodeFlow.toDescriptor(targetType);
	this.methodDescr = CodeFlow.createSignatureDescriptor(this.readMethod);
	}
	@Override
	public Class<?>[] getSpecificTargetClasses() {
	return new Class[] { this.targetType };
	}
	@Override
	public boolean canRead(EvaluationContext context, Object target, Object index) {
	return (ClassUtils.isAssignableValue(this.targetType, target) &&
	ClassUtils.isAssignableValue(this.indexType, index));
	}
	@Override
	public TypedValue read(EvaluationContext context, Object target, Object index) {
	ReflectionUtils.makeAccessible(this.readMethodToInvoke);
	Object value = ReflectionUtils.invokeMethod(this.readMethodToInvoke, target, index);
	return new TypedValue(value);
	}
	@Override
	public boolean canWrite(EvaluationContext context, Object target, Object index) {
	return false;
	}
	@Override
	public void write(EvaluationContext context, Object target, Object index, @Nullable Object newValue) {
	throw new UnsupportedOperationException();
	}
	@Override
	public boolean isCompilable() {
	return true;
	}
	@Override
	public Class<?> getIndexedValueType() {
	return this.readMethod.getReturnType();
	}
	@Override
	public void generateCode(SpelNode index, MethodVisitor mv, CodeFlow cf) {
	// Determine the public declaring class.
	Class<?> publicDeclaringClass = this.readMethodToInvoke.getDeclaringClass();
	if (!Modifier.isPublic(publicDeclaringClass.getModifiers()) && this.readMethod != null) {
	publicDeclaringClass = CodeFlow.findPublicDeclaringClass(this.readMethod);
	}
	Assert.state(publicDeclaringClass != null && Modifier.isPublic(publicDeclaringClass.getModifiers()),
	() -> "Failed to find public declaring class for: " + this.readMethod);
	// Ensure the current object on the stack is the target type.
	String lastDesc = cf.lastDescriptor();
	if (lastDesc == null || !lastDesc.equals(this.targetTypeDesc)) {
	CodeFlow.insertCheckCast(mv, this.targetTypeDesc);
	}
	// Push the index onto the stack.
	cf.generateCodeForArgument(mv, index, this.indexType);
	// Invoke the read method.
	String classDesc = publicDeclaringClass.getName().replace('.', '/');
	boolean isStatic = Modifier.isStatic(this.readMethod.getModifiers());
	boolean isInterface = publicDeclaringClass.isInterface();
	int opcode = (isStatic ? INVOKESTATIC : isInterface ? INVOKEINTERFACE : INVOKEVIRTUAL);
	mv.visitMethodInsn(opcode, classDesc, this.readMethod.getName(), this.methodDescr, isInterface);
	}
	}

Related Issues

• Introduce IndexAccessor SPI to customize the SpEL Indexer #26478
• Introduce CompilableIndexAccessor SPI in SpEL #32613

[sbrannen]

Current work on this can be viewed in the following feature branch.

main...sbrannen:spring-framework:issues/gh-32714-ReflectiveIndexAccessor

[sbrannen]

@artembilan, @pilak, @jackmiking, @jdomigon, @martin-jamszolik: you may also be interested in the new ReflectiveIndexAccessor that I just pushed to main for inclusion in Spring Framework 6.2 M2.

It greatly simplifies custom index access with zero code (plus built-in compilation support) for the most common use cases.

See commit 35c183d for details.