Last active
May 21, 2019 12:19
-
-
Save Maldivia/513d2bb27738ab957339d13af2bb5e19 to your computer and use it in GitHub Desktop.
Lazy init class using Unsafe anonymous class and Constant Dynamic from JDK11
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
package app1; | |
import org.objectweb.asm.ClassWriter; | |
import org.objectweb.asm.ConstantDynamic; | |
import org.objectweb.asm.Handle; | |
import org.objectweb.asm.Opcodes; | |
import sun.misc.Unsafe; | |
import java.lang.reflect.Field; | |
import java.util.function.Supplier; | |
public abstract class Lazy<T> implements Supplier<T> { | |
private static final String MARKER = "LAZY_MARKER"; | |
public static <T> Lazy<T> let(Supplier<T> init) { | |
ClassWriter cw = new ClassWriter(Opcodes.ASM7); | |
String lazyName = Lazy.class.getName().replace('.', '/'); | |
cw.visit(Opcodes.V11, Opcodes.ACC_PUBLIC | Opcodes.ACC_SUPER, lazyName + "$Impl", null, lazyName, null); | |
var mv = cw.visitMethod(Opcodes.ACC_PUBLIC, "<init>", "()V", null, null); | |
mv.visitCode(); | |
mv.visitVarInsn(Opcodes.ALOAD, 0); | |
mv.visitMethodInsn(Opcodes.INVOKESPECIAL, lazyName, "<init>", "()V", false); | |
mv.visitInsn(Opcodes.RETURN); | |
mv.visitMaxs(1, 1); | |
mv.visitEnd(); | |
mv = cw.visitMethod(Opcodes.ACC_PUBLIC, "get", "()Ljava/lang/Object;", null, null); | |
mv.visitCode(); | |
mv.visitLdcInsn(new ConstantDynamic("get", | |
"Ljava/lang/Object;", | |
new Handle(Opcodes.H_INVOKESTATIC, | |
"java/lang/invoke/ConstantBootstraps", | |
"invoke", | |
"(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/Class;Ljava/lang/invoke/MethodHandle;[Ljava/lang/Object;)Ljava/lang/Object;", | |
false), | |
new Handle(Opcodes.H_INVOKEINTERFACE, | |
"java/util/function/Supplier", | |
"get", | |
"()Ljava/lang/Object;", | |
true), | |
MARKER)); | |
mv.visitInsn(Opcodes.ARETURN); | |
mv.visitMaxs(1, 1); | |
mv.visitEnd(); | |
int markerIndex = cw.newConst(MARKER); | |
cw.visitEnd(); | |
byte[] bytes = cw.toByteArray(); | |
try { | |
Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe"); | |
theUnsafe.setAccessible(true); | |
Unsafe unsafe = (Unsafe) theUnsafe.get(null); | |
int cpSize = ((bytes[8] & 0xff) << 8) + (bytes[9] & 0xff); | |
Object[] cp = new Object[cpSize]; | |
cp[markerIndex] = init; | |
Class<?> aClass = unsafe.defineAnonymousClass(Lazy.class, bytes, cp); | |
return (Lazy<T>) aClass.newInstance(); | |
} | |
catch (Exception e) { | |
throw new RuntimeException(e); | |
} | |
} | |
} |
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
package app1; | |
public class MiscTest11 { | |
public static void main(String[] args) throws Exception { | |
Lazy<String> hello = Lazy.let(MiscTest11::hello); | |
System.out.println(hello.get()); | |
System.out.println(hello.get()); | |
System.out.println(hello.get()); | |
Lazy<String> world = Lazy.let(MiscTest11::world); | |
System.out.println(world.get()); | |
System.out.println(world.get()); | |
System.out.println(world.get()); | |
world = Lazy.let(MiscTest11::world); | |
System.out.println(world.get()); | |
System.out.println(world.get()); | |
System.out.println(world.get()); | |
} | |
private static String hello() { | |
System.out.println("init hello string"); | |
return "Hello"; | |
} | |
private static String world() { | |
System.out.println("init world string"); | |
return "World"; | |
} | |
} |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment