app-patching.gradle

buildscript {
    repositories {
        mavenCentral()
    }

    dependencies {
        // Some funky problems with gradle classpath management prevent us from compiling against the normal ASM dependency.
        // So we use their old '-all' shaded version since that is technically a different artifact to get access
        // to the ASM classes we need for patching.
        // 
        // When gradle is used at runtime, we use the appropriate dependency as specified in 'settings.gradle' which is
        // an up-to-date version of ASM.
        //
        // I know, gradle is dumb.
        classpath 'org.ow2.asm:asm-all:5.2'
    }
}

import org.objectweb.asm.*
import org.objectweb.asm.tree.*
import java.io.BufferedOutputStream
import java.io.File
import java.io.FileOutputStream
import java.util.Collections
import java.util.zip.ZipEntry
import java.util.zip.ZipFile
import java.util.zip.ZipOutputStream

tasks.register('patchOutput') {
    doLast {
        try {
            // We're gonna take the '-all' file and fix classes in it to be 8 compliant, and save the updated file as '-shaded'.
            // The '-all' file will be deleted when we're done.
            File file = new File(projectDir.getPath() + '/build/libs/concoction-gui-' + project.version + '-all.jar');
            File fileOut = new File(projectDir.getPath() + '/build/libs/concoction-gui-' + project.version + '-shaded.jar');
            ZipOutputStream zos = new ZipOutputStream(new BufferedOutputStream(new FileOutputStream(fileOut)));
            ZipFile zipFile = new ZipFile(file);
            for (ZipEntry entry : Collections.list(zipFile.entries())) {
                // Skip dirs, we don't need them
                if (entry.isDirectory()) continue;

                // Copy entry, downgrading Java 9+ classes when found.
                String zipEntryName = entry.getName();
                zos.putNextEntry(new ZipEntry(zipEntryName));
                byte[] zipEntryBytes = zipFile.getInputStream(entry).readAllBytes();
                if (zipEntryName.endsWith('.class') && zipEntryBytes.length > 22) {
                    // Check if version is beyond Java 8
                    byte version = zipEntryBytes[7];
                    if (version > Opcodes.V1_8) {
                        // This class uses modules for some goofy reason, even though there's no reason to.
                        // We'll rewrite the method that does this so its compliant with Java 8.
                        if (zipEntryName.contains('AbstractIkonResolver')) {
                            // Read class, patch bytes
                            ClassNode node = new ClassNode();
                            ClassReader reader = new ClassReader(zipEntryBytes);
                            reader.accept(node, ClassReader.SKIP_DEBUG);

                            // Lets rewrite this for Java 8
                            MethodNode resolveServiceLoader = node.methods.stream()
                                    .filter(m -> m.name.equals('resolveServiceLoader'))
                                    .findFirst().get();
                            InsnList i = resolveServiceLoader.instructions;
                            i.clear();
                            i.add(new LdcInsnNode(Type.getObjectType('org/kordamp/ikonli/IkonHandler')));
                            i.add(new InsnNode(Opcodes.DUP));
                            i.add(new MethodInsnNode(Opcodes.INVOKEVIRTUAL, 'java/lang/Class', 'getClassLoader', '()Ljava/lang/ClassLoader;', false));
                            i.add(new MethodInsnNode(Opcodes.INVOKESTATIC, 'java/util/ServiceLoader', 'load', '(Ljava/lang/Class;Ljava/lang/ClassLoader;)Ljava/util/ServiceLoader;', false));
                            i.add(new InsnNode(Opcodes.ARETURN));

                            // Write back patched bytes
                            ClassWriter writer = new ClassWriter(0);
                            node.accept(writer);
                            zipEntryBytes = writer.toByteArray();
                        }
                        
                        // Simple version swap will suffice
                        zipEntryBytes[7] = Opcodes.V1_8;
                    }
                    zos.write(zipEntryBytes);
                } else {
                    // Not a class, write as-is.
                    zos.write(zipEntryBytes);
                }
                zos.closeEntry();
            }
            zos.close();
            zipFile.close();
            
            // Original shaded jar no longer needed.
            file.delete();
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}