diff options
Diffstat (limited to 'tutorial/tutorial3.html')
| -rw-r--r-- | tutorial/tutorial3.html | 117 |
1 files changed, 113 insertions, 4 deletions
diff --git a/tutorial/tutorial3.html b/tutorial/tutorial3.html index dd4fb79..cb2a6b3 100644 --- a/tutorial/tutorial3.html +++ b/tutorial/tutorial3.html @@ -28,6 +28,10 @@ <p><a href="#j2me">8. J2ME</a> +<p><a href="#boxing">9. Boxing/Unboxing + +<p><a href="#debug">10. Debug</a> + <p><br> <a name="intro"> @@ -74,6 +78,28 @@ class file. <code>write()</code> in <code>ClassFile</code> writes the contents of the class file to a given <code>DataOutputStream</code>. +<p>You can create a new class file from scratch. For example, +<blockquote><pre> +ClassFile cf = new ClassFile(false, "test.Foo", null); +cf.setInterfaces(new String[] { "java.lang.Cloneable" }); + +FieldInfo f = new FieldInfo(cf.getConstPool(), "width", "I"); +f.setAccessFlags(AccessFlag.PUBLIC); +cf.addField(f); + +cf.write(new DataOutputStream(new FileOutputStream("Foo.class"))); +</pre></blockquote> + +<p>this code generates a class file <code>Foo.class</code> that contains +the implementation of the following class: + +<blockquote><pre> +package test; +class Foo implements Cloneable { + public int width; +} +</pre></blockquote> + <p><br> <a name="member"> @@ -215,6 +241,27 @@ constructed from the <code>Bytecode</code> object. To recompute the maximum stack depth of a method body, call <code>computeMaxStack()</code> in <code>CodeAttribute</code>. +<p><code>Bytecode</code> can be used to construct a method. +For example, + +<blockquote><pre> +ClassFile cf = ... +Bytecode code = new Bytecode(cf.getConstPool()); +code.addAload(0); +code.addInvokespecial("java/lang/Object", MethodInfo.nameInit, "()V"); +code.addReturn(null); +code.setMaxLocals(1); + +MethodInfo minfo = new MethodInfo(cf.getConstPool(), MethodInfo.nameInit, "()V"); +minfo.setCodeAttribute(code.toCodeAttribute()); +cf.addMethod(minfo); +</pre></blockquote> + +<p>this code makes the default constructor and adds it to the class specified +by <code>cf</code>. The <code>Bytecode</code> object is first converted into +a <code>CodeAttribute</code> object and then added to the method specified +by <code>minfo</code>. The method is finally added to a class file <code>cf</code>. + <p><br> <a name="annotation"> @@ -265,7 +312,15 @@ String s = (String)v.get(0); </pre></ul> <p>So when you write a bytecode transformer, you can just drop -off all type parameters. For example, if you have a class: +off all type parameters. Because the compiler embedded in Javassist +does not support generics, +you must insert an explicit type cast at the +caller site if the source code is compiled by Javassist, for example, +through <code>CtMethod.make()</code>. No type cast +is necessary if the source code is compiled by a normal Java compiler +such as <code>javac</code>. + +<p>For example, if you have a class: <ul><pre> public class Wrapper<T> { @@ -283,7 +338,7 @@ public interface Getter<T> { } </pre></ul> -<p>Then the interface you really have to add is <code>Getter</code> +<p>then the interface you really have to add is <code>Getter</code> (the type parameters <code><T></code> drops off) and the method you also have to add to the <code>Wrapper</code> class is this simple one: @@ -293,6 +348,23 @@ public Object get() { return value; } </pre></ul> <p>Note that no type parameters are necessary. +Since <code>get</code> returns an <code>Object</code>, an explicit type cast +is needed at the caller site if the source code is compiled by Javassist. +For example, if the type parameter <code>T</code> +is <code>String</code>, then <code>(String)</code> must be inserted as follows: + +<ul><pre> +Wrapper w = ... +String s = (String)w.get(); +</pre></ul> + +<p>The type cast is not needed if the source code is compiled by a normal Java +compiler because it will automatically insert a type cast. + +<p>If you need to make type parameters accessible through reflection +during runtime, you have to add generic signatures to the class file. +For more details, see the API documentation (javadoc) of the +<code>setGenericSignature</code> method in the <code>CtClass</code>. <p><br> @@ -318,7 +390,8 @@ cc.addMethod(m); <p>The parameter type <code>int...</code> is changed into <code>int[]</code> and <code>Modifier.VARARGS</code> is added to the method modifiers. -<p>To call this method, you must write: +<p>To call this method in the source code compiled by the compiler embedded in Javassist, +you must write: <ul><pre> length(new int[] { 1, 2, 3 }); @@ -357,10 +430,46 @@ objects, call the <code>getDeclaredMethods</code> method on a <code>CtClass</cod <p><br> +<h2><a name="boxing">9. Boxing/Unboxing</h2> + +<p>Boxing and unboxing in Java are syntactic sugar. There is no bytecode for +boxing or unboxing. So the compiler of Javassist does not support them. +For example, the following statement is valid in Java: + +<ul><pre> +Integer i = 3; +</pre></ul> + +<p>since boxing is implicitly performed. For Javassist, however, you must explicitly +convert a value type from <code>int</code> to <code>Integer</code>: + +<ul><pre> +Integer i = new Integer(3); +</pre></ul> + +<p><br> + +<h2><a name="debug">10. Debug</h2> + +<p>Set <code>CtClass.debugDump</code> to a directory name. +Then all class files modified and generated by Javassist are saved in that +directory. To stop this, set <code>CtClass.debugDump</code> to null. +The default value is null. + +<p>For example, + +<ul><pre> +CtClass.debugDump = "./dump"; +</pre></ul> + +<p>All modified class files are saved in <code>./dump</code>. + +<p><br> + <a href="tutorial2.html">Previous page</a> <hr> Java(TM) is a trademark of Sun Microsystems, Inc.<br> -Copyright (C) 2000-2010 by Shigeru Chiba, All rights reserved. +Copyright (C) 2000-2015 by Shigeru Chiba, All rights reserved. </body> </html> |