Improve names of classes generated by the SpEL compiler

Prior to this commit, the SpEL compiler generated classes in a package
named "spel" with names following the pattern "Ex#", where # was an
index starting with 2.

This resulted in class names such as:

- spel.Ex2
- spel.Ex3

This commit improves the names of classes created by the SpEL compiler
by generating classes in a package named
"org.springframework.expression.spel.generated" with names following
the pattern "CompiledExpression#####", where ##### is a 0-padded
counter starting with 00001.

This results in class names such as:

- org.springframework.expression.spel.generated.CompiledExpression00001
- org.springframework.expression.spel.generated.CompiledExpression00002

This commit also moves the saveGeneratedClassFile() method from
SpelCompilationCoverageTests to SpelCompiler and enhances it to:

- Save classes in a "build/generated-classes" directory.
- Convert package names to directories.
- Create missing parent directories.
- Use logging instead of System.out.println().

Running a test with saveGeneratedClassFile() enabled now logs something
similar to the following.

DEBUG o.s.e.s.s.SpelCompiler - Saving compiled SpEL expression [(#root.empty ? 0 : #root.size)] to [/Users/<username>/spring-framework/spring-expression/build/generated-classes/org/springframework/expression/spel/generated/CompiledExpression00001.class]

Closes gh-32497

spring-expression/src/main/java/org/springframework/expression/spel/standard/SpelCompiler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -81,7 +81,7 @@ public final class SpelCompiler implements Opcodes {
 	private volatile ChildClassLoader childClassLoader;
 
 	// Counter suffix for generated classes within this SpelCompiler instance
-	private final AtomicInteger suffixId = new AtomicInteger(1);
+	private final AtomicInteger suffixId = new AtomicInteger(0);
 
 
 	private SpelCompiler(@Nullable ClassLoader classloader) {
@@ -122,21 +122,22 @@ public CompiledExpression compile(SpelNodeImpl expression) {
 		return null;
 	}
 
-	private int getNextSuffix() {
-		return this.suffixId.incrementAndGet();
+	private String getNextSuffix() {
+		return "%05d".formatted(this.suffixId.incrementAndGet());
 	}
 
 	/**
 	 * Generate the class that encapsulates the compiled expression and define it.
-	 * The generated class will be a subtype of CompiledExpression.
+	 * <p>The generated class will be a subtype of {@link CompiledExpression}.
 	 * @param expressionToCompile the expression to be compiled
 	 * @return the expression call, or {@code null} if the decision was to opt out of
 	 * compilation during code generation
 	 */
 	@Nullable
 	private Class<? extends CompiledExpression> createExpressionClass(SpelNodeImpl expressionToCompile) {
-		// Create class outline 'spel/ExNNN extends org.springframework.expression.spel.CompiledExpression'
-		String className = "spel/Ex" + getNextSuffix();
+		// Create class outline:
+		// org.springframework.expression.spel.generated.CompiledExpression##### extends org.springframework.expression.spel.CompiledExpression
+		String className = "org/springframework/expression/spel/generated/CompiledExpression" + getNextSuffix();
 		String evaluationContextClass = "org/springframework/expression/EvaluationContext";
 		ClassWriter cw = new ExpressionClassWriter();
 		cw.visit(V1_8, ACC_PUBLIC, className, null, "org/springframework/expression/spel/CompiledExpression", null);
@@ -184,12 +185,31 @@ private Class<? extends CompiledExpression> createExpressionClass(SpelNodeImpl e
 		cf.finish();
 
 		byte[] data = cw.toByteArray();
-		// TODO Save generated class files conditionally based on a debug flag.
-		// Source code for the following method resides in SpelCompilationCoverageTests.
+		// TODO Save generated class files conditionally based on a flag.
 		// saveGeneratedClassFile(expressionToCompile.toStringAST(), className, data);
 		return loadClass(StringUtils.replace(className, "/", "."), data);
 	}
 
+	// NOTE: saveGeneratedClassFile() can be uncommented in order to review generated byte code for
+	// debugging purposes. See also: https://github.com/spring-projects/spring-framework/issues/29548
+	//
+	//	private static void saveGeneratedClassFile(String stringAST, String className, byte[] data) {
+	//		try {
+	//			// TODO Make target directory configurable.
+	//			String targetDir = "build/generated-classes";
+	//			Path path = Path.of(targetDir, className + ".class");
+	//			Files.deleteIfExists(path);
+	//			Files.createDirectories(path.getParent());
+	//			if (logger.isDebugEnabled()) {
+	//				logger.debug("Saving compiled SpEL expression [%s] to [%s]".formatted(stringAST, path.toAbsolutePath()));
+	//			}
+	//			Files.copy(new ByteArrayInputStream(data), path);
+	//		}
+	//		catch (IOException ex) {
+	//			throw new UncheckedIOException(ex);
+	//		}
+	//	}
+
 	/**
 	 * Load a compiled expression class. Makes sure the classloaders aren't used too much
 	 * because they anchor compiled classes in memory and prevent GC. If you have expressions

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

@@ -6724,20 +6724,4 @@ public void setValue2(Integer value) {
 		}
 	}
 
-	// NOTE: saveGeneratedClassFile() can be copied to SpelCompiler and uncommented
-	// at the end of createExpressionClass(SpelNodeImpl) in order to review generated
-	// byte code for debugging purposes.
-	//
-	//	private static void saveGeneratedClassFile(String stringAST, String className, byte[] data) {
-	//		try {
-	//			Path path = Path.of("build", StringUtils.replace(className, "/", ".") + ".class");
-	//			Files.deleteIfExists(path);
-	//			System.out.println("Writing compiled SpEL expression [%s] to [%s]".formatted(stringAST, path.toAbsolutePath()));
-	//			Files.copy(new ByteArrayInputStream(data), path);
-	//		}
-	//		catch (IOException ex) {
-	//			throw new UncheckedIOException(ex);
-	//		}
-	//	}
-
 }