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123 lines
6.7 KiB
HTML
123 lines
6.7 KiB
HTML
<!DOCTYPE html PUBLIC "-//w3c//dtd html 4.0 transitional//en">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="Author" content="Norris Boyd">
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<meta name="GENERATOR" content="Mozilla/4.72 [en]C-NSCP (WinNT; U) [Netscape]">
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<meta name="KeyWords" content="Rhino, JavaScript, Java">
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<title>Serialization</title>
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</head>
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<body bgcolor="#ffffff">
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<script src="owner.js"></script>
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<center>
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<h1>Serialization</h1>
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</center>
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<script>document.write(owner());</script> <br>
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<script>
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var d = new Date(document.lastModified);
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document.write((d.getMonth()+1)+"/"+d.getDate()+"/"+d.getFullYear());
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document.write('<br>');
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</script>
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<center>
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<hr width="100%"></center>
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<p>Beginning with Rhino 1.5 Release 3 it is possible to serialize JavaScript
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objects, including functions and scripts. However, serialization of
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code in compilation mode has some significant limitations.. Serialization
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provides a way to save the state of an object and write it out to a file
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or send it across a network connection. <br>
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</p>
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<h2>Simple serialization example</h2>
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The Rhino shell has two new top-level functions, serialize and deserialize.
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They're intended mainly as examples of the use of serialization:<br>
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<pre>$ java org.mozilla.javascript.tools.shell.Main<br>js> function f() { return 3; }<br>js> serialize(f, "f.ser")<br>js> quit()<br><br>$ java org.mozilla.javascript.tools.shell.Main<br>js> f = deserialize("f.ser")<br><br>function f() {<br> return 3;<br>}<br><br>js> f()<br>3<br>js></pre>
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<pre></pre>
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<pre></pre>
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<pre></pre>
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<pre></pre>
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<pre></pre>
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<pre></pre>
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<pre></pre>
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<pre></pre>
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<pre></pre>
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Here we see a simple case of a function being serialized to a file and then
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read into a new instance of Rhino and called. <br>
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<br>
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<h2>Rhino serialization APIs</h2>
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Two new classes, ScriptableOutputStream and ScriptableInputStream, were introduced
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to handle serialization of Rhino classes. These classes extend ObjectOutputStream
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and ObjectInputStream respectively. Writing an object to a file can be done
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in a few lines of Java code:<br>
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<pre>FileOutputStream fos = new FileOutputStream(filename);<br>ScriptableOutputStream out = new ScriptableOutputStream(fos, scope);<br>out.writeObject(obj);<br>out.close();</pre>
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<p>Here filename is the file to write to, obj is the object or function to
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write, and scope is the top-level scope containing obj. </p>
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<p>Reading the serialized object back into memory is similarly simple:</p>
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<pre>FileInputStream fis = new FileInputStream(filename);<br>ObjectInputStream in = new ScriptableInputStream(fis, scope);<br>Object deserialized = in.readObject();<br>in.close();<br></pre>
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<p>Again, we need the scope to create our serialization stream class. </p>
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<p>So why do we need these specialized stream classes instead of simply using
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ObjectOutputStream and ObjectInputStream? To understand the answer we must
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know what goes on behind the scenes when Rhino serializes objects. </p>
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<h2>How Rhino serialization works</h2>
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By default, Java serialization of an object also serializes objects that
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are referred to by that object. Upon deserialization the initial object and
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the objects it refers to are all created and the references between the objects
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are resolved. <br>
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<br>
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However, for JavaScript this creates a problem. JavaScript objects contain
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references to prototypes and to parent scopes. Default serialization would
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serialize the object or function we desired but would also serialize Object.prototype
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or even possibly the entire top-level scope and everything it refers to!
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We want to be able to serialize a JavaScript object and then deserialize
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it into a new scope and have all of the references from the deserialized
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object to prototypes and parent scopes resolved correctly to refer to objects
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in the new scope. <br>
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<br>
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ScriptableOutputStream takes a scope as a parameter to its constructor. If
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in the process of serialization it encounters a reference to the scope it
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will serialize a marker that will be resolved to the new scope upon deserialization.
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It is also possible to add names of objects to a list in the ScriptableOutputStream
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object. These objects will also be saved as markers upon serialization and
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resolved in the new scope upon deserialization. Use the addExcludedName method
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of ScriptableOutputStream to add new names. By default, ScriptableOutputStream
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excludes all the names defined using Context.initStandardObjects.<br>
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<br>
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If you are using Rhino serialization in an environment where you always define,
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say, a constructor "Foo", you should add the following code before calling
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writeObject:<br>
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<pre>out.addExcludedName("Foo");<br>out.addExcludedName("Foo.prototype");<br></pre>
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This code will prevent Foo and Foo.prototype from being serialized and will
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cause references to Foo or Foo.prototype to be resolved to the objects in
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the new scope upon deserialization. Exceptions will be thrown if Foo or Foo.prototype
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cannot be found the scopes used in either ScriptableOutputStream or ScriptableInputStream.<br>
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<br>
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<h2>Rhino serialization in compilation mode</h2>
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Serialization works well with objects and with functions and scripts in
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interpretive mode. However, you can run into problems with serialization
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of compiled functions and scripts:<br>
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<pre>$ cat test.js<br>function f() { return 3; }<br>serialize(f, "f.ser");<br>g = deserialize("f.ser");<br>print(g());<br>$ java org.mozilla.javascript.tools.shell.Main -opt -1 test.js<br>3<br>$ java org.mozilla.javascript.tools.shell.Main test.js<br>js: uncaught JavaScript exception: java.lang.ClassNotFoundException: c1<br></pre>
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<p>The problem is that Java serialization has no built-in way to serialize
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Java classes themselves. (It might be possible to save the Java bytecodes
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in an array and then load the class upon deserialization, but at best that
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would eat up a lot of memory for just this feature.) One way around this
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is to compile the functions using the jsc tool: </p>
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<pre>$ cat f.js<br>function f() { return 3; }<br>$ java -classpath js.jar org.mozilla.javascript.tools.jsc.Main f.js<br>$ cat test2.js<br>loadClass("f");<br>serialize(f, "f.ser");<br>g = deserialize("f.ser");<br>print(g());<br>$ java -classpath 'js.jar;.' org.mozilla.javascript.tools.shell.Main test2.js<br>3<br></pre>
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<p> Now the function f is compiled to a Java class, but that class is
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then made available in the classpath so serialization works. This isn't that
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interesting an example since compiling a function to a class and then loading
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it accomplishes the same as serializing an interpreted function, but it becomes
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more relevant if you wish to serialize JavaScript objects that have references
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to compiled functions. </p>
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<h3>
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<hr width="100%"><br>
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<a href="index.html">back to top</a>
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</h3>
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</body>
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</html>
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