|
| 1 | +import java.lang.classfile.*; |
| 2 | +import java.lang.constant.*; |
| 3 | +import java.nio.file.Files; |
| 4 | +import java.nio.file.Paths; |
| 5 | +import java.nio.file.OpenOption; |
| 6 | + |
| 7 | +/** |
| 8 | + * Run: `java --source 24 --enable-preview ClassFileApiWritingExample.java` |
| 9 | + */ |
| 10 | +public class ClassFileApiWritingExample { |
| 11 | + public static void main(String[] args) { |
| 12 | + var className = "HelloWorldFromClassFile"; |
| 13 | + |
| 14 | + var CD_System = ClassDesc.of("java.lang.System"); |
| 15 | + var CD_PrintStream = ClassDesc.of("java.io.PrintStream"); |
| 16 | + // we can use ClassDesc.of to get String class descriptor or use ConstantDescs with fixed class descriptors |
| 17 | + // arrayType returns a class descriptors for array of that type |
| 18 | + var MTD_void_StringArray = MethodTypeDesc.of(ConstantDescs.CD_void, ConstantDescs.CD_String.arrayType()); |
| 19 | + var MTD_void_String = MethodTypeDesc.of(ConstantDescs.CD_void, ClassDesc.of("java.lang.String")); |
| 20 | + |
| 21 | + System.out.println("Building class"); |
| 22 | + byte[] bytes = ClassFile.of().build( |
| 23 | + ClassDesc.of(className), |
| 24 | + clb -> clb.withFlags(ClassFile.ACC_PUBLIC) |
| 25 | + .withMethod(ConstantDescs.INIT_NAME, ConstantDescs.MTD_void, ClassFile.ACC_PUBLIC, |
| 26 | + mb -> mb.withCode( |
| 27 | + cob -> cob.aload(0) |
| 28 | + // invokes Object constructor |
| 29 | + .invokespecial(ConstantDescs.CD_Object, ConstantDescs.INIT_NAME, ConstantDescs.MTD_void) |
| 30 | + // calls System.out.println |
| 31 | + .getstatic(CD_System, "out", CD_PrintStream) |
| 32 | + .ldc("Class " + className + " constructor") |
| 33 | + .invokevirtual(CD_PrintStream, "println", MTD_void_String) |
| 34 | + .return_() |
| 35 | + ) |
| 36 | + ) |
| 37 | + // another way to build method without using MethodBuilder.withCode |
| 38 | + .withMethodBody("main", MTD_void_StringArray, ClassFile.ACC_PUBLIC + ClassFile.ACC_STATIC, |
| 39 | + cob -> cob.getstatic(CD_System, "out", CD_PrintStream) |
| 40 | + .ldc("Hello World from class built from Class File API") |
| 41 | + .invokevirtual(CD_PrintStream, "println", MTD_void_String) |
| 42 | + .return_() |
| 43 | + ) |
| 44 | + ); |
| 45 | + |
| 46 | + // writes the class to file |
| 47 | + // Inspect with: `javap HelloWorldFromClassFile.class` |
| 48 | + // Run with: `java HelloWorldFromClassFile` |
| 49 | + try { |
| 50 | + System.out.println("Writing class"); |
| 51 | + Files.write(Paths.get(className + ".class"), bytes); |
| 52 | + } catch (Exception ex) { |
| 53 | + System.err.println("Error during writing: " + ex.getMessage()); |
| 54 | + } |
| 55 | + |
| 56 | + // loads the written class |
| 57 | + try { |
| 58 | + System.out.println("Loading and running class"); |
| 59 | + var clazz = ClassLoader.getSystemClassLoader().loadClass(className); |
| 60 | + var instance = clazz.getConstructors()[0].newInstance(); |
| 61 | + clazz.getMethods()[0].invoke(instance, new Object[] { args }); |
| 62 | + } catch (ClassNotFoundException ex) { |
| 63 | + System.err.println("Class not found"); |
| 64 | + } catch (Exception ex) { |
| 65 | + System.err.println("Error during running: " + ex.getMessage()); |
| 66 | + } |
| 67 | + } |
| 68 | +} |
0 commit comments