зеркало из https://github.com/mozilla/gecko-dev.git
1493 строки
47 KiB
HTML
1493 строки
47 KiB
HTML
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{margin-top:4.0pt;
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|
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|
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|
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{margin-top:0in;
|
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|
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|
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{font-family:Courier;
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|
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{margin-top:4.0pt;
|
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|
||
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|
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|
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|
||
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{margin-top:0in;
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|
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|
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{margin-top:4.0pt;
|
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|
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|
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|
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|
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|
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|
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|
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{margin-top:4.0pt;
|
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|
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|
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|
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|
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|
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|
||
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|
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|
||
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|
||
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|
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p.SidebarTitle, li.SidebarTitle, div.SidebarTitle
|
||
{margin-top:4.0pt;
|
||
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|
||
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|
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{font-family:Times;
|
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|
||
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|
||
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|
||
span.superscript
|
||
{color:red;
|
||
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|
||
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|
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span.Symbol
|
||
{font-family:Symbol;
|
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|
||
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|
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p.TableTitle, li.TableTitle, div.TableTitle
|
||
{margin-top:12.0pt;
|
||
margin-right:.5in;
|
||
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|
||
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|
||
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|
||
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|
||
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|
||
font-family:Times;
|
||
color:navy;
|
||
font-style:italic;}
|
||
span.technicalitalic
|
||
{font-family:Times;
|
||
color:red;
|
||
font-style:italic;}
|
||
span.userinput
|
||
{font-family:Courier;
|
||
color:red;
|
||
font-weight:bold;}
|
||
span.userinputreplaceable
|
||
{font-family:Courier;
|
||
color:red;
|
||
font-weight:bold;
|
||
font-style:italic;}
|
||
/* Page Definitions */
|
||
@page Section1
|
||
{size:8.5in 11.0in;
|
||
margin:1.5in 2.0in 1.5in 2.0in;}
|
||
div.Section1
|
||
{page:Section1;}
|
||
/* List Definitions */
|
||
ol
|
||
{margin-bottom:0in;}
|
||
ul
|
||
{margin-bottom:0in;}
|
||
-->
|
||
</style>
|
||
|
||
</head>
|
||
|
||
<body lang=EN-US link=red vlink=purple>
|
||
|
||
<div class=Section1>
|
||
|
||
<p class=ChapterTitle>Scripting Java</p>
|
||
|
||
<p class=MsoNormal>This paper shows how to use Rhino to reach beyond JavaScript
|
||
into Java.<a href="#_ftn1" name="_ftnref1" title=""><span
|
||
class=MsoFootnoteReference><span class=MsoFootnoteReference><span
|
||
style='font-size:10.0pt;font-family:Times;color:black'>[1]</span></span></span></a>
|
||
Scripting Java has many uses. It allows us to write powerful scripts quickly by
|
||
making use of the many Java libraries available. We can test Java classes by
|
||
writing scripts. We can also aid our Java development by using scripting for <span
|
||
class=emphasis><span style='color:red'>exploratory programming</span></span>.
|
||
Exploratory programming is the process of learning about what a library or API
|
||
can do by writing quick programs that use it. As we will see, scripting makes
|
||
this process easier.</p>
|
||
|
||
<p class=MsoNormal>Note that the ECMA standard doesn<73>t cover communication with
|
||
Java (or with any external object system for that matter). All the
|
||
functionality covered in this chapter should thus be considered an extension.</p>
|
||
|
||
<h1>Accessing Java packages and classes</h1>
|
||
|
||
<p class=MsoNormal>Every piece of Java code is part of a class. Every Java
|
||
class is part of a package. In JavaScript, however, scripts exist outside of
|
||
any package hierarchy. How then, do we access classes in Java packages?</p>
|
||
|
||
<p class=MsoNormal>Rhino defines a top-level variable named <span
|
||
class=technicalitalic><span style='color:red'>Packages</span></span>. The
|
||
properties of the <span class=technicalitalic><span style='color:red'>Packages</span></span>
|
||
variable are all the top-level Java packages, such as <span
|
||
class=technicalitalic><span style='color:red'>java</span></span> and <span
|
||
class=technicalitalic><span style='color:red'>com</span></span>.<2E> For example,
|
||
we can access the value of the <span class=technicalitalic><span
|
||
style='color:red'>java</span></span> package:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> Packages.java</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>[JavaPackage java]</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>As a handy shortcut, Rhino defines a top-level variable <span
|
||
class=technicalitalic><span style='color:red'>java</span></span> that is
|
||
equivalent to <span class=technicalitalic><span style='color:red'>Packages.java</span></span>.
|
||
So the previous example could be even shorter:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> java</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>[JavaPackage java]</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>We can access Java classes simply by stepping down the
|
||
package hierarchy:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> java.io.File</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>[JavaClass java.io.File]</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>If your scripts access a lot of different Java classes it
|
||
can get awkward to use the full package name of the class every time. Rhino
|
||
provides a top-level function <span class=technicalitalic><span
|
||
style='color:red'>importPackage</span></span> that serves the same purpose as
|
||
Java<EFBFBD>s <span class=technicalitalic><span style='color:red'>import</span></span>
|
||
declaration. For example, we could import all of the classes in the <span
|
||
class=technicalitalic><span style='color:red'>java.io </span></span>package and
|
||
access class <span class=technicalitalic><span style='color:red'>java.io.File</span></span>
|
||
using just the name <span class=technicalitalic><span style='color:red'>File</span></span>:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js>
|
||
importPackage(java.io)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> File</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>[JavaClass java.io.File]</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>Here <span class=technicalitalic><span style='color:red'>importPackage(java.io)</span></span>
|
||
makes all the classes in the <span class=technicalitalic><span
|
||
style='color:red'>java.io</span></span> package<67> (such as <span
|
||
class=technicalitalic><span style='color:red'>File</span></span>) available at
|
||
the top level. It<49>s equivalent in effect to the Java declaration <span
|
||
class=technicalitalic><span style='color:red'>import java.io.*;</span></span>.</p>
|
||
|
||
<p class=MsoNormal>It<EFBFBD>s important to note that Java imports <span
|
||
class=technicalitalic><span style='color:red'>java.lang.*</span></span>
|
||
implicitly, while Rhino does not. The reason is that JavaScript has its own
|
||
top-level objects <span class=technicalitalic><span style='color:red'>Boolean</span></span>,
|
||
<span class=technicalitalic><span style='color:red'>Math</span></span>, <span
|
||
class=technicalitalic><span style='color:red'>Number</span></span>, <span
|
||
class=technicalitalic><span style='color:red'>Object</span></span>, and <span
|
||
class=technicalitalic><span style='color:red'>String</span></span> that are
|
||
different from the classes by those names defined in the <span
|
||
class=technicalitalic><span style='color:red'>java.lang</span></span> package.
|
||
Because of this conflict, it<69>s a good idea not to use <span
|
||
class=technicalitalic><span style='color:red'>importPackage</span></span> on
|
||
the <span class=technicalitalic><span style='color:red'>java.lang</span></span>
|
||
package. </p>
|
||
|
||
<p class=MsoNormal>One thing to be careful of is Rhino<6E>s handling of errors in
|
||
specifying Java package or class names. If <span class=technicalitalic><span
|
||
style='color:red'>java.MyClass</span></span> is accessed, Rhino attempts to
|
||
load a class named <span class=technicalitalic><span style='color:red'>java.MyClass</span></span>.
|
||
If that load fails, it assumes that <span class=technicalitalic><span
|
||
style='color:red'>java.MyClass</span></span> is a package name, and no error is
|
||
reported:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> java.MyClass</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>[JavaPackage
|
||
java.MyClass]</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>Only if you attempt to use this object as a class will an
|
||
error be reported.</p>
|
||
|
||
<h1>Working with Java objects</h1>
|
||
|
||
<p class=MsoNormal>Now that we can access Java classes, the next logical step
|
||
is to create an object. This works just as in Java, with the use of the <span
|
||
class=technicalitalic><span style='color:red'>new</span></span> operator:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> new java.util.Date()</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>Thu Jan 24 16:18:17 EST 2002</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>If we store the new object in a JavaScript variable, we can
|
||
then call methods on it:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> f = new
|
||
java.io.File("test.txt")</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>test.txt</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> f.exists()</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>true</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> f.getName()</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>test.txt</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>Static methods and fields can be accessed from the class
|
||
object itself:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> java.lang.Math.PI</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>3.141592653589793</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js>
|
||
java.lang.Math.cos(0)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>1</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>In JavaScript, unlike Java, the method by itself is an
|
||
object and can be evaluated as well as being called. If we just view the method
|
||
object by itself we can see the various overloaded forms of the method:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> f.listFiles</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>function listFiles() {/*</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>java.io.File[]
|
||
listFiles()</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>java.io.File[]
|
||
listFiles(java.io.FilenameFilter)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>java.io.File[]
|
||
listFiles(java.io.FileFilter)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>*/}</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>This output shows that the <span class=technicalitalic><span
|
||
style='color:red'>File</span></span> class defines three overloaded methods <span
|
||
class=technicalitalic><span style='color:red'>listFiles</span></span>: one that
|
||
takes no arguments, another with a <span class=technicalitalic><span
|
||
style='color:red'>FilenameFilter</span></span> argument, and a third with a <span
|
||
class=technicalitalic><span style='color:red'>FileFilter</span></span>
|
||
argument. All the methods return an array of <span class=technicalitalic><span
|
||
style='color:red'>File</span></span> objects. Being able to view the parameters
|
||
and return type of Java methods is particularly useful in exploratory
|
||
programming where we might be investigating a method and are unsure of the
|
||
parameter or return types. </p>
|
||
|
||
<p class=MsoNormal>Another useful feature for exploratory programming is the
|
||
ability to see all the methods and fields defined for an object. Using the
|
||
JavaScript <span class=technicalitalic><span style='color:red'>for..in</span></span>
|
||
construct, we can print out all these values:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> for (i in f) {
|
||
print(i) }</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>exists</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>parentFile</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>mkdir</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>toString</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>wait</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><span class=replaceable><span
|
||
style='color:red'>[44 others]</span></span></p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>Note that not only the methods of the <span
|
||
class=technicalitalic><span style='color:red'>File</span></span> class are
|
||
listed, but also the methods inherited from the base class <span
|
||
class=technicalitalic><span style='color:red'>java.lang.Object</span></span>
|
||
(like <span class=technicalitalic><span style='color:red'>wait</span></span>).
|
||
This makes it easier to work with objects in deeply nested inheritance
|
||
hierarchies since you can see all the methods that are available for that
|
||
object.</p>
|
||
|
||
<p class=MsoNormal>Rhino provides another convenience by allowing properties of
|
||
JavaBeans to be accessed directly by their property names. A JavaBean property <span
|
||
class=technicalitalic><span style='color:red'>foo</span></span> is defined by
|
||
the methods <span class=technicalitalic><span style='color:red'>getFoo</span></span>
|
||
and <span class=technicalitalic><span style='color:red'>setFoo</span></span>.
|
||
Additionally, a boolean property of the same name can be defined by an <span
|
||
class=technicalitalic><span style='color:red'>isFoo</span></span> method.<a
|
||
href="#_ftn2" name="_ftnref2" title=""><span class=MsoFootnoteReference><span
|
||
class=MsoFootnoteReference><span style='font-size:10.0pt;font-family:Times;
|
||
color:black'>[2]</span></span></span></a> For example, the following code
|
||
actually calls the <span class=technicalitalic><span style='color:red'>File</span></span>
|
||
object<EFBFBD>s <span class=technicalitalic><span style='color:red'>getName</span></span>
|
||
and <span class=technicalitalic><span style='color:red'>isDirectory</span></span>
|
||
methods.</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> f.name</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>test.txt</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> f.directory</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>false</p>
|
||
|
||
</div>
|
||
|
||
<h1>Calling overloaded methods</h1>
|
||
|
||
<p class=MsoNormal>The process of choosing a method to call based upon the
|
||
types of the arguments is called <span class=emphasis><span style='color:red'>overload
|
||
resolution</span></span>. In Java, overload resolution is performed at compile
|
||
time, while in Rhino it occurs at runtime. This difference is inevitable given
|
||
JavaScript<EFBFBD>s use of dynamic typing as was discussed in Chapter 2: since the
|
||
type of a variable is not known until runtime, only then can overload
|
||
resolution occur.</p>
|
||
|
||
<p class=MsoNormal>As an example, consider the following Java class that
|
||
defines a number of overloaded methods and calls them.</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>public class Overload {</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD> public String
|
||
f(Object o) { return "f(Object)"; }</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD> public String
|
||
f(String s) { return "f(String)"; }</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD> public String f(int
|
||
i)<29><><EFBFBD> { return "f(int)"; }</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD> public String
|
||
g(String s, int i) { </p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> return
|
||
"g(String,int)"; </p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD> }</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD> public String g(int
|
||
i, String s) { </p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> return
|
||
"g(int,String)"; </p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD> }</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD> public static void
|
||
main(String[] args) {</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> Overload o = new
|
||
Overload();</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> Object[] a = new
|
||
Object[] { new Integer(3), "hi", Overload.class };</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> for (int i=0; i
|
||
< a.length; i++)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
|
||
System.out.println(o.f(a[i]));</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD> }</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>}</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>When we compile and execute the program, it produces the
|
||
output</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>f(Object)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>f(Object)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>f(Object)</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>However, if we write a similar script </p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>var o = new
|
||
Packages.Overload()</p>
|
||
|
||
</div>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>var a = [ 3,
|
||
"hi", Packages.Overload ];</p>
|
||
|
||
</div>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>for (var i=0; i <
|
||
a.length; i++) </p>
|
||
|
||
</div>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'><EFBFBD><EFBFBD><EFBFBD> print(o.f(a[i]));</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>and execute it, we get the output</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>f(int)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>f(String)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>f(Object)</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>Because Rhino selects an overloaded method at runtime, it
|
||
calls the more specific type that matches the argument. Meanwhile Java selects
|
||
the overloaded method purely on the type of the argument at compile time. </p>
|
||
|
||
<p class=MsoNormal>Although this has the benefit of selecting a method that may
|
||
be a better match for each call, it does have an impact on performance since
|
||
more work is done at each call. In practice this performance cost hasn<73>t been
|
||
noticeable in real applications.</p>
|
||
|
||
<p class=MsoNormal>Because overload resolution occurs at runtime, it can fail
|
||
at runtime. For example, if we call <span class=technicalitalic><span
|
||
style='color:red'>Overload</span></span><EFBFBD>s method <span class=technicalitalic><span
|
||
style='color:red'>g</span></span> with two integers we get an error because
|
||
neither form of the method is closer to the argument types than the other:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> o.g(3,4)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js:
|
||
"<stdin>", line 2: The choice of Java method Overload.g
|
||
matching JavaScript argument types (number,number) is ambiguous; candidate
|
||
methods are: class java.lang.String g(java.lang.String,int), class
|
||
java.lang.String g(int,java.lang.String)</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>A more precise definition of overloading semantics can be
|
||
found at <span class=onlineitem><span style='color:red'><a
|
||
href="http://www.mozilla.org/js/liveconnect/lc3_method_overloading.html">http://www.mozilla.org/js/liveconnect/lc3_method_overloading.html</a></span></span>.</p>
|
||
|
||
<h1>Implementing Java interfaces</h1>
|
||
|
||
<p class=MsoNormal>Now that we can access Java classes, create Java objects,
|
||
and access fields, methods, and properties of those objects, we have a great
|
||
deal of power at our fingertips. However, there are a few instances where that
|
||
is not enough: many APIs in Java work by providing interfaces that clients must
|
||
implement. One example of this is the <span class=technicalitalic><span
|
||
style='color:red'>Thread</span></span> class: its constructor takes a <span
|
||
class=technicalitalic><span style='color:red'>Runnable</span></span> that
|
||
contains a single method <span class=technicalitalic><span style='color:red'>run</span></span>
|
||
that will be called when the new thread is started. </p>
|
||
|
||
<p class=MsoNormal>To address this need, Rhino provides the ability to create
|
||
new Java objects that implement interfaces. First we must define a JavaScript
|
||
object with function properties whose names match the methods required by the
|
||
Java interface. To implement a <span class=technicalitalic><span
|
||
style='color:red'>Runnable</span></span>, we need only define a single method <span
|
||
class=technicalitalic><span style='color:red'>run</span></span> with no
|
||
parameters. If you remember from Chapter 3, it is possible to define a
|
||
JavaScript object with the <span class=technicalitalic><span style='color:red'>{
|
||
</span></span><span class=replaceable><span style='color:red'>propertyName</span></span><span
|
||
class=technicalitalic><span style='color:red'>: </span></span><span
|
||
class=replaceable><span style='color:red'>value</span></span><span
|
||
class=technicalitalic><span style='color:red'> <20> }</span></span> notation. We
|
||
can use that syntax here in combination with a function expression to define a
|
||
JavaScript object with a <span class=technicalitalic><span style='color:red'>run</span></span>
|
||
method:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> obj = { run:
|
||
function () { print("\nrunning"); } }</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>[object Object]</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> obj.run()</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'> </p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>running</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>Now we can create an object implementing the <span
|
||
class=technicalitalic><span style='color:red'>Runnable</span></span> interface
|
||
by <20>constructing<6E> a <span class=technicalitalic><span style='color:red'>Runnable</span></span>:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> r = new
|
||
java.lang.Runnable(obj);</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>[object JavaObject]</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>In Java it is not possible to use the <span
|
||
class=technicalitalic><span style='color:red'>new</span></span> operator on an
|
||
interface because there is no implementation available. Here Rhino gets the
|
||
implementation from the JavaScript object <span class=technicalitalic><span
|
||
style='color:red'>obj</span></span>. Now that we have an object implementing <span
|
||
class=technicalitalic><span style='color:red'>Runnable</span></span>, we can
|
||
create a <span class=technicalitalic><span style='color:red'>Thread</span></span>
|
||
and run it. The function we defined for <span class=technicalitalic><span
|
||
style='color:red'>run </span></span>will be called on a new thread.</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> t = new
|
||
java.lang.Thread(r)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>Thread[Thread-2,5,main]</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> t.start()</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js></p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>running</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>The final <span class=literal><span style='color:red'>js</span></span>
|
||
prompt and the output from the new thread may appear in either order, depending
|
||
on thread scheduling. Rhino also allows the JavaScript object literal and the
|
||
interface creation expression to be combined for a more convenient syntax:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> r = new
|
||
java.lang.Runnable(){run: function() { print("\nrunning"); } };</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>[object JavaObject]</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>Behind the scenes, Rhino generates the bytecode for a new
|
||
Java class that implements <span class=technicalitalic><span style='color:red'>Runnable</span></span>
|
||
and forwards all calls to its <span class=technicalitalic><span
|
||
style='color:red'>run</span></span> method over to an associated JavaScript
|
||
object. The object that implements this class is called a <span class=emphasis><span
|
||
style='color:red'>Java adapter</span></span>. Because the forwarding to
|
||
JavaScript occurs at runtime, it is possible to delay defining the methods
|
||
implementing an interface until they are called. While omitting a required
|
||
method is bad practice for programming in the large, it<69>s useful for small
|
||
scripts and for exploratory programming.</p>
|
||
|
||
<h1>The <span class=technicalitalic>JavaAdapter</span> constructor</h1>
|
||
|
||
<p class=MsoNormal>In the previous section we created Java adapters using the <span
|
||
class=technicalitalic><span style='color:red'>new</span></span> operator with
|
||
Java interfaces. This approach has its limitations: it<69>s not possible to
|
||
implement multiple interfaces, nor can we extend non-abstract classes. For
|
||
these reasons there is a <span class=technicalitalic><span style='color:red'>JavaAdapter</span></span>
|
||
constructor. </p>
|
||
|
||
<p class=MsoNormal>The syntax of the <span class=technicalitalic><span
|
||
style='color:red'>JavaAdapter</span></span> constructor is</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>new JavaAdapter(<span
|
||
class=replaceable><span style='color:red'>javaIntfOrClass</span></span>, <span
|
||
class=replaceable><span style='color:red'>[javaIntfOrClass</span></span>,<span
|
||
class=replaceable><span style='color:red'> ...]</span></span> <span
|
||
class=replaceable><span style='color:red'>javascriptObject</span></span>)</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>Each <span class=replaceable><span style='color:red'>javaIntfOrClass</span></span>
|
||
is an interface to implement or a class to extend. (Only one <span
|
||
class=replaceable><span style='color:red'>javaIntfOrClass</span></span> may be
|
||
a class, of course.) The <span class=replaceable><span style='color:red'>javascriptObject</span></span>
|
||
is the JavaScript object containing the methods that will be called from the
|
||
Java adapter. </p>
|
||
|
||
<p class=MsoNormal>In practice there<72>s little need to call the <span
|
||
class=technicalitalic><span style='color:red'>JavaAdapter</span></span>
|
||
constructor directly. Most of the time the previous syntaxes using the <span
|
||
class=technicalitalic><span style='color:red'>new</span></span> operator will
|
||
be sufficient.</p>
|
||
|
||
<h1>Creating Java arrays</h1>
|
||
|
||
<p class=MsoNormal>Rhino provides no special syntax for creating Java arrays.
|
||
You must use the <span class=technicalitalic><span style='color:red'>java.lang.reflect.Array</span></span>
|
||
class for this purpose. To create an array of five Java strings you would make
|
||
the following call:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> a =
|
||
java.lang.reflect.Array.newInstance(java.lang.String, 5);</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>[Ljava.lang.String;@7ffe01</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>To create an array of primitive types, we must use the
|
||
special TYPE field defined in the associated object class in the <span
|
||
class=technicalitalic><span style='color:red'>java.lang</span></span> package.
|
||
For example, to create an array of bytes, we must use the special field <span
|
||
class=technicalitalic><span style='color:red'>java.lang.Byte.TYPE</span></span>:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> a =
|
||
java.lang.reflect.Array.newInstance(java.lang.Character.TYPE, 2);</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>[C@7a84e4</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>The resulting value can then be used anywhere a Java array
|
||
of that type is allowed.</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> a[0] = 104</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>104</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> a[1] = 105</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>105</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> new
|
||
java.lang.String(a)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>hi</p>
|
||
|
||
</div>
|
||
|
||
<h1>Java strings and JavaScript strings</h1>
|
||
|
||
<p class=MsoNormal>It<EFBFBD>s important to keep in mind that Java strings and
|
||
JavaScript strings are <span class=emphasis><span style='color:red'>not</span></span>
|
||
the same. Java strings are instances of the type <span class=technicalitalic><span
|
||
style='color:red'>java.lang.String</span></span> and have all the methods
|
||
defined by that class. JavaScript strings have methods defined by <span
|
||
class=technicalitalic><span style='color:red'>String.prototype</span></span>.
|
||
The most common stumbling block is <span class=technicalitalic><span
|
||
style='color:red'>length</span></span>, which is a method of Java strings and a
|
||
dynamic property of JavaScript strings:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> javaString = new
|
||
java.lang.String("Java")</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>Java</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> jsString =
|
||
"JavaScript"</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>JavaScript</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js>
|
||
javaString.length()</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>4</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> jsString.length</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>10</p>
|
||
|
||
</div>
|
||
|
||
<p class=MsoNormal>Rhino provides some help in reducing the differences between
|
||
the two types. First, you can pass a JavaScript string to a Java method that
|
||
requires a Java string and Rhino will perform the conversion. We actually saw
|
||
this feature in action on the call to the <span class=technicalitalic><span
|
||
style='color:red'>java.lang.String</span></span> constructor in the preceding
|
||
example.</p>
|
||
|
||
<p class=MsoNormal>Rhino also makes the JavaScript methods available to Java
|
||
strings if the java.lang.String class doesn<73>t already define them. For example:</p>
|
||
|
||
<div style='border:none;border-left:solid windowtext 1.0pt;padding:0in 0in 0in 2.0pt;
|
||
margin-left:.25in;margin-right:-67.7pt'>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>js> javaString.match(/a.*/)</p>
|
||
|
||
<p class=Code style='margin:0in;margin-bottom:.0001pt'>ava</p>
|
||
|
||
</div>
|
||
|
||
</div>
|
||
|
||
<div><br clear=all>
|
||
|
||
<hr align=left size=1 width="33%">
|
||
|
||
<div id=ftn1>
|
||
|
||
<p class=MsoFootnoteText style='margin-top:3.0pt;margin-right:67.7pt;
|
||
margin-bottom:3.0pt;margin-left:-.25in'><a href="#_ftnref1" name="_ftn1"
|
||
title=""><span class=MsoFootnoteReference><span class=MsoFootnoteReference><span
|
||
style='font-size:9.0pt;font-family:Times;color:black'>[1]</span></span></span></a>
|
||
The ability to call Java from JavaScript was first implemented as part of a
|
||
Netscape browser technology called <span class=emphasis><span style='color:
|
||
red'>LiveConnect</span></span>. However, since that technology also encompassed
|
||
communication with browser plugins, and since the way of calling JavaScript
|
||
from Java in Rhino is entirely different, that term won<6F>t be used in this paper.</p>
|
||
|
||
</div>
|
||
|
||
<div id=ftn2>
|
||
|
||
<p class=MsoFootnoteText style='margin-top:3.0pt;margin-right:67.7pt;
|
||
margin-bottom:3.0pt;margin-left:-.25in'><a href="#_ftnref2" name="_ftn2"
|
||
title=""><span class=MsoFootnoteReference><span class=MsoFootnoteReference><span
|
||
style='font-size:9.0pt;font-family:Times;color:black'>[2]</span></span></span></a>
|
||
For more information on JavaBeans, see <span class=emphasis><span
|
||
style='color:red'>Developing Java Beans</span></span> by Robert Englander.</p>
|
||
|
||
</div>
|
||
|
||
</div>
|
||
|
||
</body>
|
||
|
||
</html>
|