README.md

Class File API

JEP 457¹ Class-File API is a preview API to provide a standard API for parsing, generating, and transforming Java class files.

This repository demonstrates how to create a class that has a field, invokes a method, and prints a message, and also how to parse it with the said API.

Generating

class-file-api/src/main/java/org/jugistanbul/GenerateClass.java

Lines 20 to 35 in d0af4de

	var className = Info.CLASS_NAME.getClassName();
	ClassFile.of().buildTo(Path.of(className + ".class"), of(className),
	classBuilder -> classBuilder
	.withField("myField", of("java.lang.String"), ACC_PRIVATE)
	.withMethodBody("hello", MethodTypeDesc.of(CD_void), ACC_STATIC,
	codeBuilder -> codeBuilder
	.getstatic(of("java.lang.System"), "out", of("java.io.PrintStream"))
	.ldc("Hello, this class is generated by Class-File API which is a preview API to provide a standard API for parsing, generating, and transforming Java class files.")
	.invokevirtual(of("java.io.PrintStream"), "println",
	MethodTypeDesc.of(CD_void, of("java.lang.Object")))
	.return_())
	.withMethodBody("main", MethodTypeDesc.of(CD_void, of("java.lang.String").arrayType()), ACC_PUBLIC | ACC_STATIC,
	codeBuilder -> codeBuilder
	.invoke(Opcode.INVOKESTATIC, of(className), "hello", MethodTypeDesc.of(CD_void), false)
	.return_()));
	}

Parsing

class-file-api/src/main/java/org/jugistanbul/ParseClass.java

Lines 18 to 26 in d0af4de

	var classModel = ClassFile.of().parse(getGivenClassBytes(Info.CLASS_NAME.getClassName()));
	for (var classElement : classModel) {
	switch (classElement) {
	case MethodModel mm -> System.out.printf("Method name %s type %s%n", mm.methodName().stringValue(), mm.methodType().stringValue());
	case FieldModel fm -> System.out.printf("Field name %s type %s%n", fm.fieldName().stringValue(), fm.fieldType().stringValue());
	default -> {
	}
	}
	}

Build and run

sh run.sh

Requirements

JDK 23 or later

Footnotes

1. This proposal was refined by JEP 466 in JDK 23 and the finalized proposed version is JEP 484 targeted to JDK 24 with minor changes. ↩