Create a static constructor clone with Javassist

Question

It seems that the Javassist API allows us to create an exact copy of the class initializer (i.e. the static constructor) declared in the class:

CtClass cc = ...;
CtConstructor staticConstructor = cc.getClassInitializer();
if (staticConstructor != null) {
  CtConstructor staticConstructorClone = new CtConstructor(staticConstructor, cc, null);
  staticConstructorClone.getMethodInfo().setName(__NEW_NAME__);
  staticConstructorClone.setModifiers(Modifier.PUBLIC | Modifier.STATIC);
  cc.addConstructor(staticConstructorClone);
}

however, this copy also includes (public / private) static final fields. For example, the static constructor of the following class:

public class Example {
  public static final Example ex1 = new Example("__EX_1__");

  private String name;

  private Example(String name) {
    this.name = name;
  }
}

in fact:

static {
  Example.ex1 = "__NAME__";
}

and therefore, exactly the copy of the static constructor will also include this call in the final name field.

Is there a way to create a copy of the static constructor that does not include calls in the final fields?

- Thank

Answer 1

Introduction

, , , - ExprEditor. API Javassist , -.

, API , -, .

Example, :

public class Example {
 public static final Example ex1 = new Example("__EX_1__");
 public static String DEFAULT_NAME = "Paulo"; // <-- change 1

 private String name;

 static {
     System.out.println("Class inited");  // <-- change 2
 }

 public Example(String name) {
     this.name = name;
 }
}

, , , reset . , System.out, , , , ( ' ).

, :

public class Test {

   public static void main(String[] args) throws Throwable {
    System.out.println(Example.DEFAULT_NAME);
    Example.DEFAULT_NAME = "Jose";
    System.out.println(Example.DEFAULT_NAME);
    try {
        reset();
    } catch (Throwable t) {
        System.out.println("Problems calling reset, maybe not injected?");
    }
    System.out.println(Example.DEFAULT_NAME);
   }

   private static void reset() throws Throwable {
    Method declaredMethod = Example.class.getDeclaredMethod("__NEW_NAME__", new Class[] {});
    declaredMethod.invoke(null, new Object[] {});
   }
}

, :

Class inited
Paulo
Jose
Problems calling reset, maybe not injected?
Jose

, (, , !: P)

, , - , ? javap - Example :

javap -c -l -v -p Example.class

, .

• c: -
• l:
• v: be verbose , ..
• p:

- ( ):

 static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=3, locals=0, args_size=0
         0: new           #1                  // class test7/Example
         3: dup
         4: ldc           #13                 // String __EX_1__
         6: invokespecial #15                 // Method "<init>":(Ljava/lang/String;)V
         9: putstatic     #19                 // Field ex1:Ltest7/Example;
        12: ldc           #21                 // String Paulo
        14: putstatic     #23                 // Field DEFAULT_NAME:Ljava/lang/String;
        17: getstatic     #25                 // Field java/lang/System.out:Ljava/io/PrintStream;
        20: ldc           #31                 // String Class inited
        22: invokevirtual #33                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        25: return
      LineNumberTable:
        line 4: 0
        line 5: 12
        line 10: 17
        line 11: 25
      LocalVariableTable:
        Start  Length  Slot  Name   Signature

, , 9, a putstatic ex1, , , , .

, , , -. NEW_NAME() bytecode:

 public static void __NEW_NAME__();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=3, locals=0, args_size=0
         0: new           #1                  // class test7/Example
         3: dup
         4: ldc           #13                 // String __EX_1__
         6: invokespecial #15                 // Method "<init>":(Ljava/lang/String;)V
         9: putstatic     #19                 // Field ex1:Ltest7/Example;
        12: ldc           #21                 // String Paulo
        14: putstatic     #23                 // Field DEFAULT_NAME:Ljava/lang/String;
        17: getstatic     #25                 // Field java/lang/System.out:Ljava/io/PrintStream;
        20: ldc           #31                 // String Class inited
        22: invokevirtual #33                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        25: return
      LineNumberTable:
        line 4: 0
        line 5: 12
        line 10: 17
        line 11: 25
      LocalVariableTable:
        Start  Length  Slot  Name   Signature

- 9 , .

. , - final. , "", ? "", , - , :)

, , , , , , . :

public class Injector {

 public static void main(String[] args) throws Throwable {
    CtClass cc = ClassPool.getDefault().get(Example.class.getName());
    CtConstructor staticConstructor = cc.getClassInitializer();
    if (staticConstructor != null) {
        CtConstructor staticConstructorClone = new CtConstructor(staticConstructor, cc, null);
        staticConstructorClone.getMethodInfo().setName("__NEW_NAME__");
        staticConstructorClone.setModifiers(Modifier.PUBLIC | Modifier.STATIC);
        cc.addConstructor(staticConstructorClone);

        // Here the trick :-)

        staticConstructorClone.instrument(new ExprEditor() {

            @Override
            public void edit(FieldAccess f) throws CannotCompileException {
                try {
                    if (f.isStatic() && f.isWriter() && Modifier.isFinal(f.getField().getModifiers())) {
                        System.out.println("Found field");
                        f.replace("{  }");
                    }
                } catch (NotFoundException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        });

        cc.writeFile("...);
    }
  }
}

, , instrument ExprEditor, . , , , , "{}", " ".

- :

  public static void __NEW_NAME__();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=3, locals=2, args_size=0
         0: new           #1                  // class test7/Example
         3: dup
         4: ldc           #13                 // String __EX_1__
         6: invokespecial #15                 // Method "<init>":(Ljava/lang/String;)V
         9: astore_1
        10: aconst_null
        11: astore_0
        12: ldc           #21                 // String Paulo
        14: putstatic     #23                 // Field DEFAULT_NAME:Ljava/lang/String;
        17: getstatic     #25                 // Field java/lang/System.out:Ljava/io/PrintStream;
        20: ldc           #31                 // String Class inited
        22: invokevirtual #33                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        25: return
      LineNumberTable:
        line 4: 0
        line 5: 12
        line 10: 17
        line 11: 25
      LocalVariableTable:
        Start  Length  Slot  Name   Signature

, stackframe 9 putstatic, astore_1, javassist 3 , 9 11:

         9: astore_1
        10: aconst_null
        11: astore_0

, , :

Class inited
Paulo
Jose
Class inited
Paulo

, , , ​​ , - ... , ExprEditor, , .

resetState(), , , , , -.

, . , .