Add support for generating classes

Add `ClassGenerator` and `GeneratedClass` which can be used to
generated classes that will ultimately be written to a `JavaFile`.

See gh-28414

spring-core/src/main/java/org/springframework/aot/generate/ClassGenerator.java

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2002-2022 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.aot.generate;
+
+import java.util.Collection;
+import java.util.Collections;
+
+import org.springframework.javapoet.ClassName;
+import org.springframework.javapoet.JavaFile;
+
+/**
+ * Generates new {@link GeneratedClass} instances.
+ *
+ * @author Phillip Webb
+ * @since 6.0
+ * @see GeneratedMethods
+ */
+public interface ClassGenerator {
+
+	/**
+	 * Get or generate a new {@link GeneratedClass} for a given java file
+	 * generator, target and feature name.
+	 * @param javaFileGenerator the java file generator
+	 * @param target the target of the newly generated class
+	 * @param featureName the name of the feature that the generated class
+	 * supports
+	 * @return a {@link GeneratedClass} instance
+	 */
+	GeneratedClass getOrGenerateClass(JavaFileGenerator javaFileGenerator,
+			Class<?> target, String featureName);
+
+	/**
+	 * Get or generate a new {@link GeneratedClass} for a given java file
+	 * generator, target and feature name.
+	 * @param javaFileGenerator the java file generator
+	 * @param target the target of the newly generated class
+	 * @param featureName the name of the feature that the generated class
+	 * supports
+	 * @return a {@link GeneratedClass} instance
+	 */
+	GeneratedClass getOrGenerateClass(JavaFileGenerator javaFileGenerator, String target,
+			String featureName);
+
+
+	/**
+	 * Strategy used to generate the java file for the generated class.
+	 * Implementations of this interface are included as part of the key used to
+	 * identify classes that have already been created and as such should be
+	 * static final instances or implement a valid
+	 * {@code equals}/{@code hashCode}.
+	 */
+	@FunctionalInterface
+	interface JavaFileGenerator {
+
+		/**
+		 * Generate the file {@link JavaFile} to be written.
+		 * @param className the class name of the file
+		 * @param methods the generated methods that must be included
+		 * @return the generated files
+		 */
+		JavaFile generateJavaFile(ClassName className, GeneratedMethods methods);
+
+		/**
+		 * Return method names that must not be generated.
+		 * @return the reserved method names
+		 */
+		default Collection<String> getReservedMethodNames() {
+			return Collections.emptySet();
+		}
+
+	}
+
+}

spring-core/src/main/java/org/springframework/aot/generate/GeneratedClass.java

@@ -0,0 +1,84 @@
+/*
+ * Copyright 2002-2022 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.aot.generate;
+
+import org.springframework.aot.generate.ClassGenerator.JavaFileGenerator;
+import org.springframework.javapoet.ClassName;
+import org.springframework.javapoet.JavaFile;
+import org.springframework.util.Assert;
+
+/**
+ * A generated class.
+ *
+ * @author Phillip Webb
+ * @since 6.0
+ * @see GeneratedClasses
+ * @see ClassGenerator
+ */
+public final class GeneratedClass {
+
+	private final JavaFileGenerator JavaFileGenerator;
+
+	private final ClassName name;
+
+	private final GeneratedMethods methods;
+
+
+	/**
+	 * Create a new {@link GeneratedClass} instance with the given name. This
+	 * constructor is package-private since names should only be generated via a
+	 * {@link GeneratedClasses}.
+	 * @param name the generated name
+	 */
+	GeneratedClass(JavaFileGenerator javaFileGenerator, ClassName name) {
+		MethodNameGenerator methodNameGenerator = new MethodNameGenerator(
+				javaFileGenerator.getReservedMethodNames());
+		this.JavaFileGenerator = javaFileGenerator;
+		this.name = name;
+		this.methods = new GeneratedMethods(methodNameGenerator);
+	}
+
+
+	/**
+	 * Return the name of the generated class.
+	 * @return the name of the generated class
+	 */
+	public ClassName getName() {
+		return this.name;
+	}
+
+	/**
+	 * Return the method generator that can be used for this generated class.
+	 * @return the method generator
+	 */
+	public MethodGenerator getMethodGenerator() {
+		return this.methods;
+	}
+
+	JavaFile generateJavaFile() {
+		JavaFile javaFile = this.JavaFileGenerator.generateJavaFile(this.name,
+				this.methods);
+		Assert.state(this.name.packageName().equals(javaFile.packageName),
+				() -> "Generated JavaFile should be in package '"
+						+ this.name.packageName() + "'");
+		Assert.state(this.name.simpleName().equals(javaFile.typeSpec.name),
+				() -> "Generated JavaFile should be named '" + this.name.simpleName()
+						+ "'");
+		return javaFile;
+	}
+
+}

spring-core/src/main/java/org/springframework/aot/generate/GeneratedClasses.java

@@ -0,0 +1,94 @@
+/*
+ * Copyright 2002-2022 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.aot.generate;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+import org.springframework.util.Assert;
+
+/**
+ * A managed collection of generated classes.
+ *
+ * @author Phillip Webb
+ * @since 6.0
+ * @see GeneratedClass
+ */
+public class GeneratedClasses implements ClassGenerator {
+
+	private final ClassNameGenerator classNameGenerator;
+
+	private final Map<Owner, GeneratedClass> classes = new ConcurrentHashMap<>();
+
+
+	public GeneratedClasses(ClassNameGenerator classNameGenerator) {
+		Assert.notNull(classNameGenerator, "'classNameGenerator' must not be null");
+		this.classNameGenerator = classNameGenerator;
+	}
+
+
+	@Override
+	public GeneratedClass getOrGenerateClass(JavaFileGenerator javaFileGenerator,
+			Class<?> target, String featureName) {
+
+		Assert.notNull(javaFileGenerator, "'javaFileGenerator' must not be null");
+		Assert.notNull(target, "'target' must not be null");
+		Assert.hasLength(featureName, "'featureName' must not be empty");
+		Owner owner = new Owner(javaFileGenerator, target.getName(), featureName);
+		return this.classes.computeIfAbsent(owner,
+				key -> new GeneratedClass(javaFileGenerator,
+						this.classNameGenerator.generateClassName(target, featureName)));
+	}
+
+	@Override
+	public GeneratedClass getOrGenerateClass(JavaFileGenerator javaFileGenerator,
+			String target, String featureName) {
+
+		Assert.notNull(javaFileGenerator, "'javaFileGenerator' must not be null");
+		Assert.hasLength(target, "'target' must not be empty");
+		Assert.hasLength(featureName, "'featureName' must not be empty");
+		Owner owner = new Owner(javaFileGenerator, target, featureName);
+		return this.classes.computeIfAbsent(owner,
+				key -> new GeneratedClass(javaFileGenerator,
+						this.classNameGenerator.generateClassName(target, featureName)));
+	}
+
+	/**
+	 * Write generated Spring {@code .factories} files to the given
+	 * {@link GeneratedFiles} instance.
+	 * @param generatedFiles where to write the generated files
+	 * @throws IOException on IO error
+	 */
+	public void writeTo(GeneratedFiles generatedFiles) throws IOException {
+		Assert.notNull(generatedFiles, "'generatedFiles' must not be null");
+		List<GeneratedClass> generatedClasses = new ArrayList<>(this.classes.values());
+		generatedClasses.sort(Comparator.comparing(GeneratedClass::getName));
+		for (GeneratedClass generatedClass : generatedClasses) {
+			generatedFiles.addSourceFile(generatedClass.generateJavaFile());
+		}
+	}
+
+	private record Owner(JavaFileGenerator javaFileGenerator, String target,
+			String featureName) {
+
+	}
+
+}