Here is an example of using the FieldWriter class from the ASM library to generate Java bytecode that defines a field:

import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.FieldWriter;

public class Example {
    public static void main(String[] args) throws Exception {
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
        cw.visit(52, ACC_PUBLIC, "Example", null, "java/lang/Object", null);

        // Define a field with the following attributes:
        // - public access
        // - type: int
        // - name: field1
        FieldWriter fw = cw.visitField(ACC_PUBLIC, "field1", "I", null, null);
        fw.visitEnd();

        cw.visitEnd();

        // Get the bytecode for the Example class
        byte[] bytes = cw.toByteArray();
    }
}

This code creates a new ClassWriter instance, and then uses it to define a new class called Example. The class has a single field, field1, which is of type int and has public access. The visitField method is used to create a FieldWriter instance, which is then used to define the field. Finally, the toByteArray method is used to get the bytecode for the class.