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
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.