Basic JSNI Usage in Java

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Basic JSNI Usage
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Because JSNI isn t so well known and used, let s give a quick once over to the basic rules Calling a Java method from JavaScript requires a complex notation because your code must provide all necessary disambiguation information to distinguish among possibly overloaded methods Calls are always in the form instance@classname::method( signature)(arguments) in which Instance is the object whose method you ll be calling; you must omit it for static calls Don t forget the dot before the @ symbol! Classname is the fully qualified class name, such as comkerekinixietest clientNixieDisplay as we ll see in an upcoming example Method is the name of the method you are calling Signature is the internal Java Virtual Machine parameters signature for the method parameters, built from the following table below2 For example, if you were calling a method with (int n, String s, int[] a) parameters, the corresponding signature would be ILjava/lang/String;[I
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Arguments is the actual list of arguments that you want to pass to the called method Note that calls to vararg methods require passing an array of values3
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1 Of course, you may have to work really hard at beating the speed of GWT generated code 2 See http://javasuncom/j2se/150/docs/guide/jni/spec/typeshtml#wp276 for the specification of Java signatures 3 See http://javasuncom/j2se/150/docs/guide/language/varargshtml for more on this; the required JSNI creation of an array actually fits the old Java style for dealing with varargs
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Type
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Boolean Byte Char Double Float Integer Long Short all classes
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Signature Z B C D F I J S L fullyQualifiedClassName; with dots replaced by forward slashes, as in Ljava/lang/String; [type
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array of type
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Invoking a Java constructor follows the same rule, except that the method name is always new Accessing Java fields from JavaScript also uses a similar syntax: instance@classname::field You should take into account the following rules regarding what types can be passed back and forth between Java and JavaScript: Java String types become JavaScript string values Java Boolean types become JavaScript Boolean values Java numeric types become JavaScript number values, with the exception that long types are not supported4
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JSNI methods can return void, a Java primitive, a Java Object (created by Java code, but possibly modified by JavaScript code), or a JavaScript created JavaScriptObject; the latter, as far as GWT is concerned, will behave like a black box, and only JSNI code will be able to access it Finally, you can handle Java exceptions within JavaScript code and vice versa (JavaScript exceptions become JavaScriptException objects for Java processing) but it s recommended that you handle Java exceptions in Java code and JavaScript exceptions in JavaScript code (The reason for this is that JavaScriptException objects are untyped, which goes against the vein of usual programming You can even then get at the original exception name and description by using the getName() and getDescription() methods if you still want to process the exception yourself)
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4 JavaScript doesn t provide an adequate substitution for long variables, so they are emulated by the compiler, but this prevents using such values with JSNI See http://codegooglecom/webtoolkit/ doc/latest/DevGuideCodingBasicsJSNIhtml for further explanation
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Now, let s start out with several simple, short JSNI examples As we ll see in 10, Working with Servers, we can add a bit more security to logins and data exchanges by using hashes (A hash function takes a string as input and produces another string The important details are that there is no practical way to determine the input string from the output string, and that any change in the input string, small as it might be, will produce several changes in the output string) Hashes are usually involved in authentication protocols, and that s the way we ll use them There are many possible such functions, such as MD5 or SHA-1, and because there are quite good, free JavaScript implementations available, we can just take advantage of them5 Let s pick Paul Johnston s minified implementation of MD5 (athttp://pajhomeorguk/ crypt/md5/) and produce a JSNI wrapperYou will have to add the appropriate <script> declaration to your HTML file:
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