How to add an extra instruction to method in ByteCode using ClassVisitor / Java Bytecode (ASM)

Question

I'm writing a gradle plugin for my lib. https://github.com/shehabic/sherlock, I need to inject a network interceptor at compilation time in the byte code of OkHttp Client (https://github.com/square/okhttp/blob/master/okhttp/src/main/java/okhttp3/OkHttpClient.java) to specific I would like to inject the following line in Java:

this.interceptors.add(new com.shehabic.sherlock.interceptors(new SherlockOkHttpInterceptor())

https://github.com/square/okhttp/blob/master/okhttp/src/main/java/okhttp3/OkHttpClient.java#L1068

I have written the plugin the transformer already and here's my class writer:

public class SherlockClassWriter {

    ClassReader reader;
    ClassWriter writer;
    PublicizeMethodAdapter pubMethAdapter;
    final static String CLASSNAME = "okhttp3.OkHttpClient";

    public SherlockClassWriter() {
        try {
            reader = new ClassReader(CLASSNAME);
            writer = new ClassWriter(reader, 0);
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

    public SherlockClassWriter(byte[] contents) {
        reader = new ClassReader(contents);
        writer = new ClassWriter(reader, 0);
    }

    public static void main(String[] args) {
        SherlockClassWriter ccw = new SherlockClassWriter();
        ccw.publicizeMethod();
    }

    public byte[] publicizeMethod() {
        pubMethAdapter = new PublicizeMethodAdapter(writer);
        reader.accept(pubMethAdapter, 0);
        return writer.toByteArray();
    }

    public class PublicizeMethodAdapter extends ClassVisitor {

        TraceClassVisitor tracer;
        PrintWriter pw = new PrintWriter(System.out);

        public PublicizeMethodAdapter(ClassVisitor cv) {
            super(ASM4, cv);
            this.cv = cv;
            tracer = new TraceClassVisitor(cv, pw);
        }

        @Override
        public MethodVisitor visitMethod(
            int access,
            String name,
            String desc,
            String signature,
            String[] exceptions
        ) {
            if (name.equals("build")) {
                MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);
                // call method in java:
                // this.interceptors.add(new com.shehabic.sherlock.interceptors(new SherlockOkHttpInterceptor())
            }
            return tracer.visitMethod(access, name, desc, signature, exceptions);
        }
    }
}

a similar method that adds interceptors has a bytecode as follows:

aload_0
getfield #4 <okhttp3/OkHttpClient$Builder.interceptors>
aload_1
invokeinterface #117 <java/util/List.add> count 2
pop
aload_0

My questions are: 1.How do I inject more code into a method? even if Bytecode.

Update Here is my working solution, based on the answer: https://github.com/shehabic/sherlock/blob/creating-plugin-to-intercept-all-okhttp-connections/sherlock-plugin/src/main/java/com/shehabic/sherlock/plugin/SherlockClassWriter.java

asm.ow2.io/asm4-guide.pdf is a great resource for this sort of stuff. I'd recommend using the Tree API (using MethodNode) as it makes it much easier to actually see what's happening. — Matthew Kerian
– Matthew Kerian, Commented Jan 15, 2019 at 11:45

Юрий Баринов · Accepted Answer · 2019-01-21 12:18:08Z

There is example code to insert your line at the beggining of the function

public class YourClassVisitor extends ClassVisitor {
    public YourClassVisitor(ClassVisitor cv) {
        super(Opcodes.ASM5, cv);
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
        if (name.equals("targetName")) {
            return new YourMethodVisitor(super.visitMethod(access, name, desc, signature, exceptions));
        }
        return super.visitMethod(access, name, desc, signature, exceptions);
    }


    private static class YourMethodVisitor extends MethodVisitor {
        public YourMethodVisitor(MethodVisitor mv) {
            super(Opcodes.ASM5, mv);
        }

        // This method will be called before almost all instructions
        @Override
        public void visitCode() {
            // Default implementation is empty. So we haven't to call super method

            // Puts 'this' on top of the stack. If your method is static just delete it
            visitVarInsn(Opcodes.ALOAD, 0);
            // Takes instance of class "the/full/name/of/your/Class" from top of the stack and put value of field interceptors
            // "Ljava/util/List;" is just internal name of java.util.List
            // If your field is static just replace GETFIELD with GETSTATIC
            visitFieldInsn(Opcodes.GETFIELD, "the/full/name/of/your/Class", "interceptors", "Ljava/util/List;");
            // Before we call add method of list we have to put target value on top of the stack
            // New object creation starts with creating not initialized instance of it
            visitTypeInsn(Opcodes.NEW, "com/shehabic/sherlock/interceptors");
            // Than we just copy it
            visitInsn(Opcodes.DUP);
            visitTypeInsn(Opcodes.NEW, "example/path/to/class/SherlockOkHttpInterceptor");
            visitInsn(Opcodes.DUP);
            // We have to call classes constructor
            // Internal name of constructor - <init>
            // ()V - signature of method. () - method doesn't have parameters. V - method returns void
            visitMethodInsn(Opcodes.INVOKESPECIAL, "example/path/to/class/SherlockOkHttpInterceptor", "<init>", "()V", false);
            // So on top of the stack we have initialized instance of example/path/to/class/SherlockOkHttpInterceptor
            // Now we can call constructor of com/shehabic/sherlock/interceptors
            visitMethodInsn(Opcodes.INVOKESPECIAL, "com/shehabic/sherlock/interceptors", "<init>", "(Lexample/path/to/class/SherlockOkHttpInterceptor;)V", false);
            // So on top of the stack we have initialized instance of com/shehabic/sherlock/interceptors
            // Now we can put it into list
            visitMethodInsn(Opcodes.INVOKEINTERFACE, "java/util/List", "add", "(Ljava/lang/Object;)Z", true);



        }
    }

}

There is example of using class visitor

        byte[] cache = null;
        try (FileInputStream in = new FileInputStream("C:\\Users\\JustAGod\\Projects\\gloomymods\\BuildTools\\BytecodeTools\\out\\production\\classes\\gloomyfolken\\Kek.class")) {
            ClassWriter writer = new ClassWriter(ClassWriter.COMPUTE_MAXS);
            ClassReader reader = new ClassReader(in);
            reader.accept(new YourClassVisitor(writer), ClassReader.EXPAND_FRAMES);
            cache = writer.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
        }
        try(FileOutputStream out = new FileOutputStream("C:\\Users\\JustAGod\\Projects\\gloomymods\\BuildTools\\BytecodeTools\\out\\production\\classes\\gloomyfolken\\Kek.class")) {
            out.write(cache);
        } catch (IOException e) {
            e.printStackTrace();
        }

I am really sorry for my English.

I'll try that and if it works I'll mark it as the right answer

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