Make SpelNode compilation aware

In order to make SpelNode compilation aware, this method moves the
declaration of isCompilable() and generateCode(...) from SpelNodeImpl
to SpelNode.

Closes gh-32707

Author: sbrannen

Diff for: spring-expression/src/main/java/org/springframework/expression/spel/SpelNode.java

@@ -16,6 +16,7 @@
 
 package org.springframework.expression.spel;
 
+import org.springframework.asm.MethodVisitor;
 import org.springframework.expression.EvaluationException;
 import org.springframework.expression.TypedValue;
 import org.springframework.lang.Nullable;
@@ -25,6 +26,7 @@
  * Expression Language (SpEL) expression.
  *
  * @author Andy Clement
+ * @author Sam Brannen
  * @since 3.0
  */
 public interface SpelNode {
@@ -109,4 +111,40 @@ public interface SpelNode {
 	 */
 	int getEndPosition();
 
+	/**
+	 * Determine if this node can be compiled to bytecode.
+	 * <p>The reasoning in each node may be different but will typically involve
+	 * checking whether the exit type descriptor of the node is known and any
+	 * relevant child nodes are compilable.
+	 * <p>The default implementation returns {@code false}.
+	 * <p>If you override this method, you must also override
+	 * {@link #generateCode(MethodVisitor, CodeFlow)}.
+	 * @return {@code true} if this node can be compiled to bytecode
+	 * @since 6.2
+	 * @see #generateCode(MethodVisitor, CodeFlow)
+	 */
+	default boolean isCompilable() {
+		return false;
+	}
+
+	/**
+	 * Generate the bytecode for this node into the supplied {@link MethodVisitor}.
+	 * <p>Context information about the current expression being compiled is
+	 * available in the supplied {@link CodeFlow} object &mdash; for example,
+	 * information about the type of the object currently on the stack.
+	 * <p>This method will not be invoked unless {@link #isCompilable()} returns
+	 * {@code true}.
+	 * <p>The default implementation throws an {@link IllegalStateException}
+	 * since {@link #isCompilable()} returns {@code false} by default.
+	 * <p>If you override this method, you must also override {@link #isCompilable()}.
+	 * @param methodVisitor the ASM {@code MethodVisitor} into which code should
+	 * be generated
+	 * @param codeFlow a context object with information about what is on the stack
+	 * @since 6.2
+	 * @see #isCompilable()
+	 */
+	default void generateCode(MethodVisitor methodVisitor, CodeFlow codeFlow) {
+		throw new IllegalStateException(getClass().getName() + " does not support bytecode generation");
+	}
+
 }

Diff for: spring-expression/src/main/java/org/springframework/expression/spel/ast/SpelNodeImpl.java

@@ -191,27 +191,6 @@ public boolean isNullSafe() {
 		return false;
 	}
 
-	/**
-	 * Check whether a node can be compiled to bytecode. The reasoning in each node may
-	 * be different but will typically involve checking whether the exit type descriptor
-	 * of the node is known and any relevant child nodes are compilable.
-	 * @return {@code true} if this node can be compiled to bytecode
-	 */
-	public boolean isCompilable() {
-		return false;
-	}
-
-	/**
-	 * Generate the bytecode for this node into the supplied visitor. Context info about
-	 * the current expression being compiled is available in the codeflow object, e.g.
-	 * including information about the type of the object currently on the stack.
-	 * @param mv the ASM MethodVisitor into which code should be generated
-	 * @param cf a context object with info about what is on the stack
-	 */
-	public void generateCode(MethodVisitor mv, CodeFlow cf) {
-		throw new IllegalStateException(getClass().getName() +" has no generateCode(..) method");
-	}
-
 	@Nullable
 	public String getExitDescriptor() {
 		return this.exitTypeDescriptor;