JDK 24: example of Class File API

Author: wesleyegberto

java-24/BasicClass.java

@@ -0,0 +1,43 @@
+import java.io.Serializable;
+
+/**
+ * BasicClass
+ */
+public class BasicClass implements Serializable {
+	private String attribute;
+	private int timestamp;
+	private boolean active;
+
+	public BasicClass() {
+	}
+
+	public BasicClass(String attribute, int timestamp, boolean active) {
+		this.attribute = attribute;
+		this.timestamp = timestamp;
+		this.active = active;
+	}
+
+	public String getAttribute() {
+		return attribute;
+	}
+
+	public void setAttribute(String attribute) {
+		this.attribute = attribute;
+	}
+
+	public int getTimestamp() {
+		return timestamp;
+	}
+
+	public void setTimestamp(int timestamp) {
+		this.timestamp = timestamp;
+	}
+
+	public boolean isActive() {
+		return active;
+	}
+
+	public void setActive(boolean active) {
+		this.active = active;
+	}
+}

java-24/ClassFileApiReadingExample.java

@@ -0,0 +1,52 @@
+import java.lang.classfile.*;
+import java.lang.classfile.constantpool.ClassEntry;
+import java.lang.constant.*;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.util.stream.Collectors;
+
+/**
+ * Run: `javac BasicClass.java && java --source 24 --enable-preview ClassFileApiReadingExample.java`
+ */
+public class ClassFileApiReadingExample {
+	public static void main(String[] args) throws Exception {
+		var classBytes = Files.readAllBytes(Paths.get("BasicClass.class"));
+
+		// ClassModel is an immutable description of a class file
+		ClassModel cm = ClassFile.of().parse(classBytes);
+
+		// ClasModel is lazy, iterating over it parses the entire class
+		for (ClassElement ce : cm) {
+			// possible values: https://download.java.net/java/early_access/jdk24/docs/api/java.base/java/lang/classfile/ClassElement.html
+			switch (ce) {
+				case Superclass cn -> System.out.println("Superclass: " + cn.superclassEntry().name().stringValue());
+
+				case Interfaces i -> {
+					var interfaces = i.interfaces().stream()
+							.map(ClassEntry::name)
+							.collect(Collectors.joining(","));
+					System.out.println("Interfaces: " + interfaces);
+				}
+
+				// ClassModel.fields()
+				case FieldModel fm -> {
+					var fieldType = fm.fieldTypeSymbol().displayName();
+					var fieldName = fm.fieldName().stringValue();
+					System.out.printf("Field: %s %s%n", fieldType, fieldName);
+				}
+
+				// ClassModel.methods()
+				case MethodModel mm -> {
+					var symbol = mm.methodTypeSymbol();
+					var returnType = symbol.returnType().displayName();
+					var parameters = symbol.parameterList().stream()
+							.map(ClassDesc::displayName)
+							.collect(Collectors.joining(","));
+					var methodName = mm.methodName().stringValue();
+					System.out.printf("Method: %s %s(%s)%n", returnType, methodName, parameters);
+				}
+				default -> System.out.printf("Other: %s%n", ce);
+			}
+		}
+	}
+}

java-24/README.md

@@ -92,9 +92,11 @@ To run each example use: `java --enable-preview --source 24 <FileName.java>`
         ```java
         ClassModel cm = ClassFile.of().parse(bytes);
         for (ClassElement ce : cm) {
-            case FieldModel fm -> System.out.println("Field " + mm.methodName().stringValue());
-            case MethodModel mm -> System.out.println("Method " + mm.methodName().stringValue());
-            default -> System.out.println("Other " + ce);
+            switch (ce) {
+                case FieldModel fm -> System.out.println("Field " + fm.fieldName().stringValue());
+                case MethodModel mm -> System.out.println("Method " + mm.methodName().stringValue());
+                default -> System.out.println("Other " + ce);
+            }
         }
         ```
     * `ClassFile` also provides methods to access the elements we need (parsing only the part need to get the methods):
@@ -153,7 +155,7 @@ To run each example use: `java --enable-preview --source 24 <FileName.java>`
     * re-preview with no change
     * introduced new terminology "simple source file" to indicate a Java file with a implicitly declared class
 * **Structured Concurrency**
-    * 
+    * re-preview with no change
 
 ## Links