Getting Started with Javassist

Shigeru Chiba

Next page

1. Reading and writing bytecode
2. ClassPool
3. Class loader
4. Introspection and customization
5. Bytecode level API
6. Generics
7. Varargs
8. J2ME
9. Boxing/Unboxing
10. Debug

## 1. Reading and writing bytecode

Javassist is a class library for dealing with Java bytecode. Java bytecode is stored in a binary file called a class file. Each class file contains one Java class or interface.

The class `Javassist.CtClass` is an absatract representation of a class file. A `CtClass` (compile-time class) object is a handle for dealing with a class file. The following program is a very simple example:

    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("test.Rectangle");
    cc.setSuperclass(pool.get("test.Point"));
    cc.writeFile();

This program first obtains a `ClassPool` object, which controls bytecode modification with Javassist. The `ClassPool` object is a container of `CtClass` object representing a class file. It reads a class file on demand for constructing a `CtClass` object and records the constructed object for responding later accesses. To modify the definition of a class, the users must first obtain from a `ClassPool` object a reference to a `CtClass` object representing that class. `get()` in `ClassPool` is used for this purpose. In the case of the program shown above, the `CtClass` object representing a class `test.Rectangle` is obtained from the `ClassPool` object and it is assigned to a variable `cc`. The `ClassPool` object returned by `getDfault()` searches the default system search path.

From the implementation viewpoint, `ClassPool` is a hash table of `CtClass` objects, which uses the class names as keys. `get()` in `ClassPool` searches this hash table to find a `CtClass` object associated with the specified key. If such a `CtClass` object is not found, `get()` reads a class file to construct a new `CtClass` object, which is recorded in the hash table and then returned as the resulting value of `get()`.

The `CtClass` object obtained from a `ClassPool` object can be modified (details of how to modify a `CtClass` will be presented later). In the example above, it is modified so that the superclass of `test.Rectangle` is changed into a class `test.Point`. This change is reflected on the original class file when `writeFile()` in `CtClass()` is finally called.

`writeFile()` translates the `CtClass` object into a class file and writes it on a local disk. Javassist also provides a method for directly obtaining the modified bytecode. To obtain the bytecode, call `toBytecode()`:

    byte[] b = cc.toBytecode();

You can directly load the `CtClass` as well:

    Class clazz = cc.toClass();

`toClass()` requests the context class loader for the current thread to load the class file represented by the `CtClass`. It returns a `java.lang.Class` object representing the loaded class. For more details, please see this section below.

#### Defining a new class

To define a new class from scratch, `makeClass()` must be called on a `ClassPool`.

    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.makeClass("Point");

This program defines a class `Point` including no members. Member methods of `Point` can be created with factory methods declared in `CtNewMethod` and appended to `Point` with `addMethod()` in `CtClass`.

`makeClass()` cannot create a new interface; `makeInterface()` in `ClassPool` can do. Member methods in an interface can be created with `abstractMethod()` in `CtNewMethod`. Note that an interface method is an abstract method.

#### Frozen classes

If a `CtClass` object is converted into a class file by `writeFile()`, `toClass()`, or `toBytecode()`, Javassist freezes that `CtClass` object. Further modifications of that `CtClass` object are not permitted. This is for warning the developers when they attempt to modify a class file that has been already loaded since the JVM does not allow reloading a class.

A frozen `CtClass` can be defrost so that modifications of the class definition will be permitted. For example,

    CtClasss cc = ...;
        :
    cc.writeFile();
    cc.defrost();
    cc.setSuperclass(...);    // OK since the class is not frozen.

After `defrost()` is called, the `CtClass` object can be modified again.

If `ClassPool.doPruning` is set to `true`, then Javassist prunes the data structure contained in a `CtClass` object when Javassist freezes that object. To reduce memory consumption, pruning discards unnecessary attributes (`attribute_info` structures) in that object. For example, `Code_attribute` structures (method bodies) are discarded. Thus, after a `CtClass` object is pruned, the bytecode of a method is not accessible except method names, signatures, and annotations. The pruned `CtClass` object cannot be defrost again. The default value of `ClassPool.doPruning` is `false`.

To disallow pruning a particular `CtClass`, `stopPruning()` must be called on that object in advance:

    CtClasss cc = ...;
    cc.stopPruning(true);
        :
    cc.writeFile();                             // convert to a class file.
    // cc is not pruned.

The `CtClass` object `cc` is not pruned. Thus it can be defrost after `writeFile()` is called.

Note: While debugging, you might want to temporarily stop pruning and freezing and write a modified class file to a disk drive. `debugWriteFile()` is a convenient method for that purpose. It stops pruning, writes a class file, defrosts it, and turns pruning on again (if it was initially on).

#### Class search path

The default `ClassPool` returned by a static method `ClassPool.getDefault()` searches the same path that the underlying JVM (Java virtual machine) has. If a program is running on a web application server such as JBoss and Tomcat, the `ClassPool` object may not be able to find user classes since such a web application server uses multiple class loaders as well as the system class loader. In that case, an additional class path must be registered to the `ClassPool`. Suppose that `pool` refers to a `ClassPool` object:

    pool.insertClassPath(new ClassClassPath(this.getClass()));

This statement registers the class path that was used for loading the class of the object that `this` refers to. You can use any `Class` object as an argument instead of `this.getClass()`. The class path used for loading the class represented by that `Class` object is registered.

You can register a directory name as the class search path. For example, the following code adds a directory `/usr/local/javalib` to the search path:

    ClassPool pool = ClassPool.getDefault();
    pool.insertClassPath("/usr/local/javalib");

The search path that the users can add is not only a directory but also a URL:

    ClassPool pool = ClassPool.getDefault();
    ClassPath cp = new URLClassPath("www.javassist.org", 80, "/java/", "org.javassist.");
    pool.insertClassPath(cp);

This program adds "http://www.javassist.org:80/java/" to the class search path. This URL is used only for searching classes belonging to a package `org.javassist`. For example, to load a class `org.javassist.test.Main`, its class file will be obtained from:

    http://www.javassist.org:80/java/org/javassist/test/Main.class

Furthermore, you can directly give a byte array to a `ClassPool` object and construct a `CtClass` object from that array. To do this, use `ByteArrayClassPath`. For example,

    ClassPool cp = ClassPool.getDefault();
    byte[] b = a byte array;
    String name = class name;
    cp.insertClassPath(new ByteArrayClassPath(name, b));
    CtClass cc = cp.get(name);

The obtained `CtClass` object represents a class defined by the class file specified by `b`. The `ClassPool` reads a class file from the given `ByteArrayClassPath` if `get()` is called and the class name given to `get()` is equal to one specified by `name`.

If you do not know the fully-qualified name of the class, then you can use `makeClass()` in `ClassPool`:

    ClassPool cp = ClassPool.getDefault();
    InputStream ins = an input stream for reading a class file;
    CtClass cc = cp.makeClass(ins);

`makeClass()` returns the `CtClass` object constructed from the given input stream. You can use `makeClass()` for eagerly feeding class files to the `ClassPool` object. This might improve performance if the search path includes a large jar file. Since a `ClassPool` object reads a class file on demand, it might repeatedly search the whole jar file for every class file. `makeClass()` can be used for optimizing this search. The `CtClass` constructed by `makeClass()` is kept in the `ClassPool` object and the class file is never read again.

The users can extend the class search path. They can define a new class implementing `ClassPath` interface and give an instance of that class to `insertClassPath()` in `ClassPool`. This allows a non-standard resource to be included in the search path.

## 2. ClassPool

A `ClassPool` object is a container of `CtClass` objects. Once a `CtClass` object is created, it is recorded in a `ClassPool` for ever. This is because a compiler may need to access the `CtClass` object later when it compiles source code that refers to the class represented by that `CtClass`.

For example, suppose that a new method `getter()` is added to a `CtClass` object representing `Point` class. Later, the program attempts to compile source code including a method call to `getter()` in `Point` and use the compiled code as the body of a method, which will be added to another class `Line`. If the `CtClass` object representing `Point` is lost, the compiler cannot compile the method call to `getter()`. Note that the original class definition does not include `getter()`. Therefore, to correctly compile such a method call, the `ClassPool` must contain all the instances of `CtClass` all the time of program execution.

#### Avoid out of memory

This specification of `ClassPool` may cause huge memory consumption if the number of `CtClass` objects becomes amazingly large (this rarely happens since Javassist tries to reduce memory consumption in various ways). To avoid this problem, you can explicitly remove an unnecessary `CtClass` object from the `ClassPool`. If you call `detach()` on a `CtClass` object, then that `CtClass` object is removed from the `ClassPool`. For example,

    CtClass cc = ... ;
    cc.writeFile();
    cc.detach();

You must not call any method on that `CtClass` object after `detach()` is called. However, you can call `get()` on `ClassPool` to make a new instance of `CtClass` representing the same class. If you call `get()`, the `ClassPool` reads a class file again and newly creates a `CtClass` object, which is returned by `get()`.

Another idea is to occasionally replace a `ClassPool` with a new one and discard the old one. If an old `ClassPool` is garbage collected, the `CtClass` objects included in that `ClassPool` are also garbage collected. To create a new instance of `ClassPool`, execute the following code snippet:

    ClassPool cp = new ClassPool(true);
    // if needed, append an extra search path by appendClassPath()

This creates a `ClassPool` object that behaves as the default `ClassPool` returned by `ClassPool.getDefault()` does. Note that `ClassPool.getDefault()` is a singleton factory method provided for convenience. It creates a `ClassPool` object in the same way shown above although it keeps a single instance of `ClassPool` and reuses it. A `ClassPool` object returned by `getDefault()` does not have a special role. `getDefault()` is a convenience method.

Note that `new ClassPool(true)` is a convenient constructor, which constructs a `ClassPool` object and appends the system search path to it. Calling that constructor is equivalent to the following code:

    ClassPool cp = new ClassPool();
    cp.appendSystemPath();  // or append another path by appendClassPath()

#### Cascaded ClassPools

If a program is running on a web application server, creating multiple instances of `ClassPool` might be necessary; an instance of `ClassPool` should be created for each class loader (i.e. container). The program should create a `ClassPool` object by not calling `getDefault()` but a constructor of `ClassPool`.

Multiple `ClassPool` objects can be cascaded like `java.lang.ClassLoader`. For example,

    ClassPool parent = ClassPool.getDefault();
    ClassPool child = new ClassPool(parent);
    child.insertClassPath("./classes");

If `child.get()` is called, the child `ClassPool` first delegates to the parent `ClassPool`. If the parent `ClassPool` fails to find a class file, then the child `ClassPool` attempts to find a class file under the `./classes` directory.

If `child.childFirstLookup` is true, the child `ClassPool` attempts to find a class file before delegating to the parent `ClassPool`. For example,

    ClassPool parent = ClassPool.getDefault();
    ClassPool child = new ClassPool(parent);
    child.appendSystemPath();         // the same class path as the default one.
    child.childFirstLookup = true;    // changes the behavior of the child.

#### Changing a class name for defining a new class

A new class can be defined as a copy of an existing class. The program below does that:

    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("Point");
    cc.setName("Pair");

This program first obtains the `CtClass` object for class `Point`. Then it calls `setName()` to give a new name `Pair` to that `CtClass` object. After this call, all occurrences of the class name in the class definition represented by that `CtClass` object are changed from `Point` to `Pair`. The other part of the class definition does not change.

Note that `setName()` in `CtClass` changes a record in the `ClassPool` object. From the implementation viewpoint, a `ClassPool` object is a hash table of `CtClass` objects. `setName()` changes the key associated to the `CtClass` object in the hash table. The key is changed from the original class name to the new class name.

Therefore, if `get("Point")` is later called on the `ClassPool` object again, then it never returns the `CtClass` object that the variable `cc` refers to. The `ClassPool` object reads a class file `Point.class` again and it constructs a new `CtClass` object for class `Point`. This is because the `CtClass` object associated with the name `Point` does not exist any more. See the followings:

    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("Point");
    CtClass cc1 = pool.get("Point");   // cc1 is identical to cc.
    cc.setName("Pair");
    CtClass cc2 = pool.get("Pair");    // cc2 is identical to cc.
    CtClass cc3 = pool.get("Point");   // cc3 is not identical to cc.

`cc1` and `cc2` refer to the same instance of `CtClass` that `cc` does whereas `cc3` does not. Note that, after `cc.setName("Pair")` is executed, the `CtClass` object that `cc` and `cc1` refer to represents the `Pair` class.

The `ClassPool` object is used to maintain one-to-one mapping between classes and `CtClass` objects. Javassist never allows two distinct `CtClass` objects to represent the same class unless two independent `ClassPool` are created. This is a significant feature for consistent program transformation.

To create another copy of the default instance of `ClassPool`, which is returned by `ClassPool.getDefault()`, execute the following code snippet (this code was already shown above):

    ClassPool cp = new ClassPool(true);

If you have two `ClassPool` objects, then you can obtain, from each `ClassPool`, a distinct `CtClass` object representing the same class file. You can differently modify these `CtClass` objects to generate different versions of the class.

#### Renaming a frozen class for defining a new class

Once a `CtClass` object is converted into a class file by `writeFile()` or `toBytecode()`, Javassist rejects further modifications of that `CtClass` object. Hence, after the `CtClass` object representing `Point` class is converted into a class file, you cannot define `Pair` class as a copy of `Point` since executing `setName()` on `Point` is rejected. The following code snippet is wrong:

    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("Point");
    cc.writeFile();
    cc.setName("Pair");    // wrong since writeFile() has been called.

To avoid this restriction, you should call `getAndRename()` in `ClassPool`. For example,

    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("Point");
    cc.writeFile();
    CtClass cc2 = pool.getAndRename("Point", "Pair");

If `getAndRename()` is called, the `ClassPool` first reads `Point.class` for creating a new `CtClass` object representing `Point` class. However, it renames that `CtClass` object from `Point` to `Pair` before it records that `CtClass` object in a hash table. Thus `getAndRename()` can be executed after `writeFile()` or `toBytecode()` is called on the the `CtClass` object representing `Point` class.

## 3. Class loader

If what classes must be modified is known in advance, the easiest way for modifying the classes is as follows:

- 1. Get a `CtClass` object by calling `ClassPool.get()`,
- 2. Modify it, and
- 3. Call `writeFile()` or `toBytecode()` on that `CtClass` object to obtain a modified class file.

If whether a class is modified or not is determined at load time, the users must make Javassist collaborate with a class loader. Javassist can be used with a class loader so that bytecode can be modified at load time. The users of Javassist can define their own version of class loader but they can also use a class loader provided by Javassist.

### 3.1 The `toClass` method in `CtClass`

The `CtClass` provides a convenience method `toClass()`, which requests the context class loader for the current thread to load the class represented by the `CtClass` object. To call this method, the caller must have appropriate permission; otherwise, a `SecurityException` may be thrown.

The following program shows how to use `toClass()`:

    public class Hello {
        public void say() {
            System.out.println("Hello");
        }
    }

    public class Test {
        public static void main(String[] args) throws Exception {
            ClassPool cp = ClassPool.getDefault();
            CtClass cc = cp.get("Hello");
            CtMethod m = cc.getDeclaredMethod("say");
            m.insertBefore("{ System.out.println(\"Hello.say():\"); }");
            Class c = cc.toClass();
            Hello h = (Hello)c.newInstance();
            h.say();
        }
    }

`Test.main()` inserts a call to `println()` in the method body of `say()` in `Hello`. Then it constructs an instance of the modified `Hello` class and calls `say()` on that instance.

Note that the program above depends on the fact that the `Hello` class is never loaded before `toClass()` is invoked. If not, the JVM would load the original `Hello` class before `toClass()` requests to load the modified `Hello` class. Hence loading the modified `Hello` class would be failed (`LinkageError` is thrown). For example, if `main()` in `Test` is something like this:

    public static void main(String[] args) throws Exception {
        Hello orig = new Hello();
        ClassPool cp = ClassPool.getDefault();
        CtClass cc = cp.get("Hello");
            :
    }

then the original `Hello` class is loaded at the first line of `main` and the call to `toClass()` throws an exception since the class loader cannot load two different versions of the `Hello` class at the same time.

If the program is running on some application server such as JBoss and Tomcat, the context class loader used by `toClass()` might be inappropriate. In this case, you would see an unexpected `ClassCastException`. To avoid this exception, you must explicitly give an appropriate class loader to `toClass()`. For example, if `bean` is your session bean object, then the following code:

    CtClass cc = ...;
    Class c = cc.toClass(bean.getClass().getClassLoader());

would work. You should give `toClass()` the class loader that has loaded your program (in the above example, the class of the `bean` object).

`toClass()` is provided for convenience. If you need more complex functionality, you should write your own class loader.

### 3.2 Class loading in Java

In Java, multiple class loaders can coexist and each class loader creates its own name space. Different class loaders can load different class files with the same class name. The loaded two classes are regarded as different ones. This feature enables us to run multiple application programs on a single JVM even if these programs include different classes with the same name.

Note: The JVM does not allow dynamically reloading a class. Once a class loader loads a class, it cannot reload a modified version of that class during runtime. Thus, you cannot alter the definition of a class after the JVM loads it. However, the JPDA (Java Platform Debugger Architecture) provides limited ability for reloading a class. See Section 3.6.

If the same class file is loaded by two distinct class loaders, the JVM makes two distinct classes with the same name and definition. The two classes are regarded as different ones. Since the two classes are not identical, an instance of one class is not assignable to a variable of the other class. The cast operation between the two classes fails and throws a `ClassCastException`.

For example, the following code snippet throws an exception:

    MyClassLoader myLoader = new MyClassLoader();
    Class clazz = myLoader.loadClass("Box");
    Object obj = clazz.newInstance();
    Box b = (Box)obj;    // this always throws ClassCastException.

The `Box` class is loaded by two class loaders. Suppose that a class loader CL loads a class including this code snippet. Since this code snippet refers to `MyClassLoader`, `Class`, `Object`, and `Box`, CL also loads these classes (unless it delegates to another class loader). Hence the type of the variable `b` is the `Box` class loaded by CL. On the other hand, `myLoader` also loads the `Box` class. The object `obj` is an instance of the `Box` class loaded by `myLoader`. Therefore, the last statement always throws a `ClassCastException` since the class of `obj` is a different verison of the `Box` class from one used as the type of the variable `b`.

Multiple class loaders form a tree structure. Each class loader except the bootstrap loader has a parent class loader, which has normally loaded the class of that child class loader. Since the request to load a class can be delegated along this hierarchy of class loaders, a class may be loaded by a class loader that you do not request the class loading. Therefore, the class loader that has been requested to load a class C may be different from the loader that actually loads the class C. For distinction, we call the former loader the initiator of C and we call the latter loader the real loader of C.

Furthermore, if a class loader CL requested to load a class C (the initiator of C) delegates to the parent class loader PL, then the class loader CL is never requested to load any classes referred to in the definition of the class C. CL is not the initiator of those classes. Instead, the parent class loader PL becomes their initiators and it is requested to load them. The classes that the definition of a class C referes to are loaded by the real loader of C.

To understand this behavior, let's consider the following example.

    public class Point {    // loaded by PL
        private int x, y;
        public int getX() { return x; }
            :
    }

    public class Box {      // the initiator is L but the real loader is PL
        private Point upperLeft, size;
        public int getBaseX() { return upperLeft.x; }
            :
    }

    public class Window {    // loaded by a class loader L
        private Box box;
        public int getBaseX() { return box.getBaseX(); }
    }

Suppose that a class `Window` is loaded by a class loader L. Both the initiator and the real loader of `Window` are L. Since the definition of `Window` refers to `Box`, the JVM will request L to load `Box`. Here, suppose that L delegates this task to the parent class loader PL. The initiator of `Box` is L but the real loader is PL. In this case, the initiator of `Point` is not L but PL since it is the same as the real loader of `Box`. Thus L is never requested to load `Point`.

Next, let's consider a slightly modified example.

    public class Point {
        private int x, y;
        public int getX() { return x; }
            :
    }

    public class Box {      // the initiator is L but the real loader is PL
        private Point upperLeft, size;
        public Point getSize() { return size; }
            :
    }

    public class Window {    // loaded by a class loader L
        private Box box;
        public boolean widthIs(int w) {
            Point p = box.getSize();
            return w == p.getX();
        }
    }

Now, the definition of `Window` also refers to `Point`. In this case, the class loader L must also delegate to PL if it is requested to load `Point`. You must avoid having two class loaders doubly load the same class. One of the two loaders must delegate to the other.

If L does not delegate to PL when `Point` is loaded, `widthIs()` would throw a ClassCastException. Since the real loader of `Box` is PL, `Point` referred to in `Box` is also loaded by PL. Therefore, the resulting value of `getSize()` is an instance of `Point` loaded by PL whereas the type of the variable `p` in `widthIs()` is `Point` loaded by L. The JVM regards them as distinct types and thus it throws an exception because of type mismatch.

This behavior is somewhat inconvenient but necessary. If the following statement:

    Point p = box.getSize();

did not throw an exception, then the programmer of `Window` could break the encapsulation of `Point` objects. For example, the field `x` is private in `Point` loaded by PL. However, the `Window` class could directly access the value of `x` if L loads `Point` with the following definition:

    public class Point {
        public int x, y;    // not private
        public int getX() { return x; }
            :
    }

For more details of class loaders in Java, the following paper would be helpful:

Sheng Liang and Gilad Bracha, "Dynamic Class Loading in the Java Virtual Machine",
ACM OOPSLA'98, pp.36-44, 1998.

### 3.3 Using `javassist.Loader`

Javassist provides a class loader `javassist.Loader`. This class loader uses a `javassist.ClassPool` object for reading a class file.

For example, `javassist.Loader` can be used for loading a particular class modified with Javassist.

    import javassist.*;
    import test.Rectangle;

    public class Main {
      public static void main(String[] args) throws Throwable {
         ClassPool pool = ClassPool.getDefault();
         Loader cl = new Loader(pool);

         CtClass ct = pool.get("test.Rectangle");
         ct.setSuperclass(pool.get("test.Point"));

         Class c = cl.loadClass("test.Rectangle");
         Object rect = c.newInstance();
             :
      }
    }

This program modifies a class `test.Rectangle`. The superclass of `test.Rectangle` is set to a `test.Point` class. Then this program loads the modified class, and creates a new instance of the `test.Rectangle` class.

If the users want to modify a class on demand when it is loaded, the users can add an event listener to a `javassist.Loader`. The added event listener is notified when the class loader loads a class. The event-listener class must implement the following interface:

    public interface Translator {
        public void start(ClassPool pool)
            throws NotFoundException, CannotCompileException;
        public void onLoad(ClassPool pool, String classname)
            throws NotFoundException, CannotCompileException;
    }

The method `start()` is called when this event listener is added to a `javassist.Loader` object by `addTranslator()` in `javassist.Loader`. The method `onLoad()` is called before `javassist.Loader` loads a class. `onLoad()` can modify the definition of the loaded class.

For example, the following event listener changes all classes to public classes just before they are loaded.

    public class MyTranslator implements Translator {
        void start(ClassPool pool)
            throws NotFoundException, CannotCompileException {}
        void onLoad(ClassPool pool, String classname)
            throws NotFoundException, CannotCompileException
        {
            CtClass cc = pool.get(classname);
            cc.setModifiers(Modifier.PUBLIC);
        }
    }

Note that `onLoad()` does not have to call `toBytecode()` or `writeFile()` since `javassist.Loader` calls these methods to obtain a class file.

To run an application class `MyApp` with a `MyTranslator` object, write a main class as following:

    import javassist.*;

    public class Main2 {
      public static void main(String[] args) throws Throwable {
         Translator t = new MyTranslator();
         ClassPool pool = ClassPool.getDefault();
         Loader cl = new Loader();
         cl.addTranslator(pool, t);
         cl.run("MyApp", args);
      }
    }

To run this program, do:

    % java Main2 arg1 arg2...

The class `MyApp` and the other application classes are translated by `MyTranslator`.

Note that application classes like `MyApp` cannot access the loader classes such as `Main2`, `MyTranslator`, and `ClassPool` because they are loaded by different loaders. The application classes are loaded by `javassist.Loader` whereas the loader classes such as `Main2` are by the default Java class loader.

`javassist.Loader` searches for classes in a different order from `java.lang.ClassLoader`. `ClassLoader` first delegates the loading operations to the parent class loader and then attempts to load the classes only if the parent class loader cannot find them. On the other hand, `javassist.Loader` attempts to load the classes before delegating to the parent class loader. It delegates only if:

- the classes are not found by calling `get()` on a `ClassPool` object, or

- the classes have been specified by using `delegateLoadingOf()` to be loaded by the parent class loader.

This search order allows loading modified classes by Javassist. However, it delegates to the parent class loader if it fails to find modified classes for some reason. Once a class is loaded by the parent class loader, the other classes referred to in that class will be also loaded by the parent class loader and thus they are never modified. Recall that all the classes referred to in a class C are loaded by the real loader of C. If your program fails to load a modified class, you should make sure whether all the classes using that class have been loaded by `javassist.Loader`.

### 3.4 Writing a class loader

A simple class loader using Javassist is as follows:

    import javassist.*;

    public class SampleLoader extends ClassLoader {
        /* Call MyApp.main().
         */
        public static void main(String[] args) throws Throwable {
            SampleLoader s = new SampleLoader();
            Class c = s.loadClass("MyApp");
            c.getDeclaredMethod("main", new Class[] { String[].class })
             .invoke(null, new Object[] { args });
        }

        private ClassPool pool;

        public SampleLoader() throws NotFoundException {
            pool = new ClassPool();
            pool.insertClassPath("./class"); // MyApp.class must be there.
        }

        /* Finds a specified class.
         * The bytecode for that class can be modified.
         */
        protected Class findClass(String name) throws ClassNotFoundException {
            try {
                CtClass cc = pool.get(name);
                // modify the CtClass object here
                byte[] b = cc.toBytecode();
                return defineClass(name, b, 0, b.length);
            } catch (NotFoundException e) {
                throw new ClassNotFoundException();
            } catch (IOException e) {
                throw new ClassNotFoundException();
            } catch (CannotCompileException e) {
                throw new ClassNotFoundException();
            }
        }
    }

The class `MyApp` is an application program. To execute this program, first put the class file under the `./class` directory, which must not be included in the class search path. Otherwise, `MyApp.class` would be loaded by the default system class loader, which is the parent loader of `SampleLoader`. The directory name `./class` is specified by `insertClassPath()` in the constructor. You can choose a different name instead of `./class` if you want. Then do as follows:

`% java SampleLoader`

The class loader loads the class `MyApp` (`./class/MyApp.class`) and calls `MyApp.main()` with the command line parameters.

This is the simplest way of using Javassist. However, if you write a more complex class loader, you may need detailed knowledge of Java's class loading mechanism. For example, the program above puts the `MyApp` class in a name space separated from the name space that the class `SampleLoader` belongs to because the two classes are loaded by different class loaders. Hence, the `MyApp` class cannot directly access the class `SampleLoader`.

### 3.5 Modifying a system class

The system classes like `java.lang.String` cannot be loaded by a class loader other than the system class loader. Therefore, `SampleLoader` or `javassist.Loader` shown above cannot modify the system classes at loading time.

If your application needs to do that, the system classes must be statically modified. For example, the following program adds a new field `hiddenValue` to `java.lang.String`:

    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("java.lang.String");
    CtField f = new CtField(CtClass.intType, "hiddenValue", cc);
    f.setModifiers(Modifier.PUBLIC);
    cc.addField(f);
    cc.writeFile(".");

This program produces a file `"./java/lang/String.class"`.

To run your program `MyApp` with this modified `String` class, do as follows:

    % java -Xbootclasspath/p:. MyApp arg1 arg2...

Suppose that the definition of `MyApp` is as follows:

    public class MyApp {
        public static void main(String[] args) throws Exception {
            System.out.println(String.class.getField("hiddenValue").getName());
        }
    }

If the modified `String` class is correctly loaded, `MyApp` prints `hiddenValue`.

Note: Applications that use this technique for the purpose of overriding a system class in `rt.jar` should not be deployed as doing so would contravene the Java 2 Runtime Environment binary code license.

### 3.6 Reloading a class at runtime

If the JVM is launched with the JPDA (Java Platform Debugger Architecture) enabled, a class is dynamically reloadable. After the JVM loads a class, the old version of the class definition can be unloaded and a new one can be reloaded again. That is, the definition of that class can be dynamically modified during runtime. However, the new class definition must be somewhat compatible to the old one. The JVM does not allow schema changes between the two versions. They have the same set of methods and fields.

Javassist provides a convenient class for reloading a class at runtime. For more information, see the API documentation of `javassist.tools.HotSwapper`.

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