查看当前类的依赖
假如有一个HelloWorld类：
import java.util.Random;
public class HelloWorld {
public static void main(String[] args) throws InterruptedException {
Random rand = new Random();
int val = rand.nextInt(1000);
Thread.sleep(val);
System.out.println("Hello World");
}
}
查看HelloWorld类所依赖的类：
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.commons.ClassRemapper;
import org.objectweb.asm.commons.Remapper;
import java.util.HashSet;
import java.util.Set;
public class HelloWorldAnalysisCore {
public static void main(String[] args) {
String relative_path = "sample/HelloWorld.class";
String filepath = FileUtils.getFilePath(relative_path);
byte[] bytes = FileUtils.readBytes(filepath);
//（1）构建ClassReader
ClassReader cr = new ClassReader(bytes);
//（2）分析ClassVisitor
Set<String> dependencies = new HashSet<>();
EmptyClassVisitor emptyVisitor = new EmptyClassVisitor(Opcodes.ASM9, null);
ClassVisitor cv = new ClassRemapper(emptyVisitor, new Remapper() {
@Override
public String map(String internalName) {
dependencies.add(internalName);
return super.map(internalName);
}
});
//（3）结合ClassReader和ClassVisitor
int parsingOptions = ClassReader.SKIP_DEBUG | ClassReader.SKIP_FRAMES;
cr.accept(cv, parsingOptions);
//（4）打印依赖信息
for (String item : dependencies) {
System.out.println(item);
}
}
}
输出结果：
java/lang/Object
java/lang/String
java/util/Random
sample/HelloWorld
java/lang/InterruptedException
java/lang/System
java/lang/Thread
java/io/PrintStream
代码分析
核心代码段
Set<String> dependencies = new HashSet<>();
EmptyClassVisitor emptyVisitor = new EmptyClassVisitor(Opcodes.ASM9, null);
ClassVisitor cv = new ClassRemapper(emptyVisitor, new Remapper() {
@Override
public String map(String internalName) {
dependencies.add(internalName);
return super.map(internalName);
}
});
EmptyClassVisitor
对于EmptyClassVisitor类，它本身并没有做什么有用的操作，只是一个空的实现，有三点情况需要注意：
- 第一点，可以将
EmptyClassVisitor类实例替换成ClassWriter类实例，但是没有很大的必要。原因：ClassWriter类功能更强大，但这些强大的功能，在当前场景下，并不需要使用到。 - 第二点，不能将
EmptyClassVisitor类实例替换成匿名类new ClassVisitor(Opcodes.ASM9) {}。原因：在默认情况下，ClassVisitor的visitField()和visitMethod()返回值为null，影响功能发挥。 - 第三点，不能将
EmptyClassVisitor类实例替换成null，影响功能正常发挥。
import org.objectweb.asm.*;
public class EmptyClassVisitor extends ClassVisitor {
public EmptyClassVisitor(int api, ClassVisitor classVisitor) {
super(api, classVisitor);
}
@Override
public ModuleVisitor visitModule(String name, int access, String version) {
ModuleVisitor mv = super.visitModule(name, access, version);
return new EmptyModuleVisitor(api, mv);
}
@Override
public AnnotationVisitor visitAnnotation(String descriptor, boolean visible) {
AnnotationVisitor av = super.visitAnnotation(descriptor, visible);
return new EmptyAnnotationVisitor(api, av);
}
@Override
public AnnotationVisitor visitTypeAnnotation(int typeRef, TypePath typePath, String descriptor, boolean visible) {
AnnotationVisitor av = super.visitTypeAnnotation(typeRef, typePath, descriptor, visible);
return new EmptyAnnotationVisitor(api, av);
}
@Override
public RecordComponentVisitor visitRecordComponent(String name, String descriptor, String signature) {
RecordComponentVisitor rcv = super.visitRecordComponent(name, descriptor, signature);
return new EmptyRecordComponentVisitor(api, rcv);
}
@Override
public FieldVisitor visitField(int access, String name, String descriptor, String signature, Object value) {
FieldVisitor fv = super.visitField(access, name, descriptor, signature, value);
return new EmptyFieldVisitor(api, fv);
}
@Override
public MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
MethodVisitor mv = super.visitMethod(access, name, descriptor, signature, exceptions);
return new EmptyMethodVisitor(api, mv);
}
}
ClassRemapper和Remapper
在下面代码中，我们可以看到ClassRemapper和Remapper类，这两个类一般用于“混淆处理.class文件”，两者有各自的作用：
ClassRemapper类负责找出所有可以替换的位置（类名、方法名和字段名）。Remapper类负责决定哪些位置需要进行替换。
Set<String> dependencies = new HashSet<>();
EmptyClassVisitor emptyVisitor = new EmptyClassVisitor(Opcodes.ASM9, null);
ClassVisitor cv = new ClassRemapper(emptyVisitor, new Remapper() {
@Override
public String map(String internalName) {
dependencies.add(internalName);
return super.map(internalName);
}
});
在ClassRemapper类当中，我们重点关注于Remapper.mapType(String)方法，它会再进一步调用Remapper.map(String)方法。在记录类的依赖关系时，正是在Remapper.map(String)方法中进行的。
另外，如果我们有兴趣，可以进一步查看MethodRemapper.visitMethodInsn()方法。
public class ClassRemapper extends ClassVisitor {
protected final Remapper remapper;
public ClassRemapper(final ClassVisitor classVisitor, final Remapper remapper) {
this(Opcodes.ASM9, classVisitor, remapper);
}
protected ClassRemapper(final int api, final ClassVisitor classVisitor, final Remapper remapper) {
super(api, classVisitor);
this.remapper = remapper;
}
@Override
public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
this.className = name;
super.visit(version, access,
remapper.mapType(name),
remapper.mapSignature(signature, false),
remapper.mapType(superName),
interfaces == null ? null : remapper.mapTypes(interfaces));
}
@Override
public MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
String remappedDescriptor = remapper.mapMethodDesc(descriptor);
MethodVisitor methodVisitor =
super.visitMethod(access,
remapper.mapMethodName(className, name, descriptor),
remappedDescriptor,
remapper.mapSignature(signature, false),
exceptions == null ? null : remapper.mapTypes(exceptions));
return methodVisitor == null ? null : createMethodRemapper(methodVisitor);
}
protected MethodVisitor createMethodRemapper(final MethodVisitor methodVisitor) {
return new MethodRemapper(api, methodVisitor, remapper);
}
}