JDK 24: added example of Class File API transformation

Author: wesleyegberto

java-21/README.md

@@ -340,4 +340,5 @@ Deprecations:
   * [Java 21 Brings Full Pattern Matching](https://www.youtube.com/watch?v=QrwFrm1R8OY)
   * [Java's Virtual Threads - Next Steps](https://www.youtube.com/watch?v=KBW4LbCoo6c)
   * [Continuations: The magic behind virtual threads in Java by Balkrishna Rawool @ Spring I/O 2024](https://www.youtube.com/watch?v=pwLtYvRK334)
+  * [Sequenced Collections - Deep Dive with the Expert](https://www.youtube.com/watch?v=6yuDqkkYTGU)
 

java-24/BasicClass.java

@@ -1,5 +1,8 @@
 import java.io.Serializable;
 
+/**
+ * Used in Class File API examples.
+ */
 public class BasicClass implements Serializable {
 	private String attribute;
 	private int timestamp;

java-24/ClassFileApiReadingExample.java

@@ -6,7 +6,7 @@
 import java.util.stream.Collectors;
 
 /**
- * Run: `javac BasicClass.java && java --source 24 --enable-preview ClassFileApiReadingExample.java`
+ * Run: `javac BasicClass.java && java ClassFileApiReadingExample.java`
  */
 public class ClassFileApiReadingExample {
 	public static void main(String[] args) throws Exception {

java-24/ClassFileApiTestTransformedBasicClass.java

@@ -0,0 +1,12 @@
+/**
+ * Used in Class File API transforming example.
+ */
+public class ClassFileApiTestTransformedBasicClass {
+	public static void main(String[] args) {
+		System.out.println("Using the transformed BasicClass");
+		var obj = new BasicClass("The next arg will not be returned", 10203040, true);
+		// won't exists after transformation
+		// obj.setTimestamp(123);
+		System.out.println("Return of getTimestamp: " + obj.getTimestamp());
+	}
+}

java-24/ClassFileApiTransformingExample.java

@@ -0,0 +1,81 @@
+import java.lang.classfile.*;
+import java.lang.constant.*;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+
+/**
+ * CodeBuilder doc: https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/lang/classfile/CodeBuilder.html
+ *
+ * - compile BasicClass and ClassFileApiTestTransformedBasicClass classes
+ * - run transformation code in the BasicClass bytes
+ * - run BasicClass testing class
+ *
+ * Run: `javac ClassFileApiTestTransformedBasicClass.java && java ClassFileApiTransformingExample.java && java ClassFileApiTestTransformedBasicClass`
+ */
+public class ClassFileApiTransformingExample {
+	public static void main(String[] args) throws Exception {
+		var classBytes = Files.readAllBytes(Paths.get("BasicClass.class"));
+
+		var transformedClassBytes = dropMethod(classBytes);
+		transformedClassBytes = transformMethod(transformedClassBytes);
+
+		writeFile(transformedClassBytes);
+	}
+
+	static byte[] dropMethod(byte[] classBytes) {
+		System.out.println("Dropping setters");
+		ClassModel originalClassModel = ClassFile.of().parse(classBytes);
+		byte[] transformedClassBytes = ClassFile.of().build(
+				originalClassModel.thisClass().asSymbol(),
+				classBuilder -> {
+					for (ClassElement ce : originalClassModel) {
+						// removes every setter
+						if (!(ce instanceof MethodModel mm && mm.methodName().stringValue().startsWith("set"))) {
+							classBuilder.with(ce);
+						}
+					}
+				}
+		);
+
+		// there are helpful methods in ClassFile to do the transformations
+		var classfile = ClassFile.of();
+		transformedClassBytes = classfile.transformClass(
+				classfile.parse(classBytes),
+				ClassTransform.dropping(ce -> ce instanceof MethodModel mm && mm.methodName().stringValue().startsWith("set"))
+		);
+		return transformedClassBytes;
+	}
+
+	static byte[] transformMethod(byte[] classBytes) {
+		System.out.println("Transforming timestamp getter");
+		ClassModel originalClassModel = ClassFile.of().parse(classBytes);
+		byte[] transformedClassBytes = ClassFile.of().build(
+				originalClassModel.thisClass().asSymbol(),
+				cb -> {
+					for (ClassElement ce : originalClassModel) {
+						// replaces the getTimestamp method
+						if (ce instanceof MethodModel mm && mm.methodName().stringValue().equals("getTimestamp")) {
+							var mtdInt = MethodTypeDesc.of(ConstantDescs.CD_int);
+							cb.withFlags()
+									.withMethodBody(
+											mm.methodName().stringValue(), mtdInt, ClassFile.ACC_PUBLIC,
+											cob -> cob.ldc(42).ireturn()
+									);
+						} else {
+							cb.with(ce);
+						}
+					}
+				}
+		);
+		return transformedClassBytes;
+	}
+
+	static void writeFile(byte[] bytes) {
+		System.out.println("Writing BasicClass file");
+		try {
+			Files.write(Paths.get("BasicClass.class"), bytes);
+		} catch (Exception ex) {
+			System.err.println("Error during writing: " + ex.getMessage());
+		}
+	}
+}

java-24/ClassFileApiWritingExample.java

@@ -5,7 +5,7 @@
 import java.nio.file.OpenOption;
 
 /**
- * Run: `java --source 24 --enable-preview ClassFileApiWritingExample.java`
+ * Run: `java ClassFileApiWritingExample.java`
  */
 public class ClassFileApiWritingExample {
 	public static void main(String[] args) {