gradle-master/subprojects/normalization-java/src/main/java/org/gradle/internal/normalization/java/impl/MethodStubbingApiMemberAdapter.java
/*
 * Copyright 2016 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.gradle.internal.normalization.java.impl;

import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

import static org.objectweb.asm.Opcodes.ACC_ABSTRACT;
import static org.objectweb.asm.Opcodes.ACC_INTERFACE;
import static org.objectweb.asm.Opcodes.ACC_PRIVATE;
import static org.objectweb.asm.Opcodes.ACC_STATIC;
import static org.objectweb.asm.Opcodes.ACC_SYNTHETIC;
import static org.objectweb.asm.Opcodes.ARETURN;
import static org.objectweb.asm.Opcodes.ATHROW;
import static org.objectweb.asm.Opcodes.DUP;
import static org.objectweb.asm.Opcodes.INVOKESPECIAL;
import static org.objectweb.asm.Opcodes.INVOKESTATIC;
import static org.objectweb.asm.Opcodes.NEW;

/**
 * Adapts members selected by {@link ApiMemberSelector}, stripping out method
 * implementations and replacing them with a "stub" that will throw an
 * {@link UnsupportedOperationException} if called at runtime. All members (including but
 * not limited to stripped and stubbed methods) are delegated to a {@link ClassWriter}
 * responsible for writing new API classes.
 */
public class MethodStubbingApiMemberAdapter extends ClassVisitor {

    private static final String UOE_METHOD = "$unsupportedOpEx";

    private String internalClassName;

    public MethodStubbingApiMemberAdapter(ClassWriter cv) {
        super(Opcodes.ASM9, cv);
    }

    @Override
    public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
        super.visit(version, access, name, signature, superName, interfaces);
        internalClassName = name;
        if ((access & ACC_INTERFACE) == 0) {
            generateUnsupportedOperationExceptionMethod();
        }
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
        MethodVisitor mv = cv.visitMethod(access, name, desc, signature, exceptions);
        if ((access & ACC_ABSTRACT) == 0) {
            mv.visitCode();
            mv.visitMethodInsn(
                INVOKESTATIC, internalClassName, UOE_METHOD, "()Ljava/lang/UnsupportedOperationException;", false);
            mv.visitInsn(ATHROW);
            mv.visitMaxs(1, 0);
            mv.visitEnd();
        }
        return mv;
    }

    /**
     * Generates an exception which is going to be thrown in each method.
     * The reason it is in a separate method is because it reduces the bytecode size.
     */
    private void generateUnsupportedOperationExceptionMethod() {
        MethodVisitor mv = cv.visitMethod(
            ACC_PRIVATE | ACC_STATIC | ACC_SYNTHETIC, UOE_METHOD,
            "()Ljava/lang/UnsupportedOperationException;", null, null);
        mv.visitCode();
        mv.visitTypeInsn(NEW, "java/lang/UnsupportedOperationException");
        mv.visitInsn(DUP);
        mv.visitLdcInsn(
            "You tried to call a method on an API class. Is the API jar on the classpath instead of the runtime jar?");
        mv.visitMethodInsn(
            INVOKESPECIAL, "java/lang/UnsupportedOperationException", "<init>", "(Ljava/lang/String;)V", false);
        mv.visitInsn(ARETURN);
        mv.visitMaxs(3, 0);
        mv.visitEnd();
    }
}
