distro-source/core/src/doc/helpInfo/jsoRestrictions.html - gwt - Git at Google

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 <head>
 <meta http-equiv="content-type" content="text/html; charset=UTF-8">
 <title>Restrictions on subclasses of JavaScriptObject</title>
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 <body>
 <p>Subclasses of JavaScriptObject represent a view of a JavaScript
 object from Java. Such classes must conform to a number of restrictions
 so that the compiler can implement them. This page lists those
 restrictions.</p>
 <p>In the following, "JSO class" means any subclass of <code>JavaScriptObject</code>.
 Rationales are written <em>like this</em>.</p>
 <ol>
   <li>All instance methods on JSO classes must be one of:
   explicitly final, a member of a final class, or private. <em>Methods
   of JSO classes cannot be overridden, because calls to such methods
   could require dynamic dispatch.</em></li>
   <li>An interface type may be implemented by at most one JSO
   subtype. <em>This ensures that polymorphic dispatch via a
   "SingleJsoImpl" interface can be statically resolved to exactly one
   implementing JSO subtype.</em>
   <ol>
     <li>A <code>JavaScriptObject</code> that implements a
     "SingleJsoImpl" interface may be further extended. The subclasses
     may implement additional "SingleJsoImpl" interfaces. <em>The
     methods on a JSO must be effectively final, so each "SingleJsoImpl"
     method still has a 1:1 mapping to a method defined within a JSO
     subtype.</em></li>
     <li>It is valid for any number of any non-<code>JavaScriptObject</code>
     types to implement a "SingleJsoImpl" interface. <em>There is a
     slight runtime dispatch penalty when a "SingleJsoImpl" interface is
     implemented by both JSO and non-JSO types.</em></li>
   </ol>
   </li>
   <li>No instance methods on JSO classes may override another
   method. <em>This catches accidents where JSO itself did not
   finalize some method from its superclass.</em></li>
   <li>JSO classes cannot have instance fields. <em>The fields
   would have no place to live in web mode. Programmers should instead
   make an explicit wrapper class and put the fields there.</em></li>
   <li>Nested JSO classes must be static. <em>The implicit <code>this</code>
   fields of a non-static inner class has the same problems as an
   explicit field.</em></li>
   <li>"new" operations cannot be used with JSO classes. <em>This
   avoids ever being able to try to instantiate JSO objects using the new
   keyword. New JSO instances can only come from JSNI, as in previous
   versions of GWT.</em></li>
   <li>JSNI methods may not refer to instance methods defined within
   a <code>JavaScriptObject</code>.</li>
   <li>Every JSO class must have precisely one constructor, and it
   must be protected, empty, and no-argument.</li>
 </ol>
 </body>
 </html>
	<html>
	<head>
	<meta http-equiv="content-type" content="text/html; charset=UTF-8">
	<title>Restrictions on subclasses of JavaScriptObject</title>
	</head>
	<body>
	<p>Subclasses of JavaScriptObject represent a view of a JavaScript
	object from Java. Such classes must conform to a number of restrictions
	so that the compiler can implement them. This page lists those
	restrictions.</p>
	<p>In the following, "JSO class" means any subclass of <code>JavaScriptObject</code>.
	Rationales are written <em>like this</em>.</p>
	<ol>
	<li>All instance methods on JSO classes must be one of:
	explicitly final, a member of a final class, or private. <em>Methods
	of JSO classes cannot be overridden, because calls to such methods
	could require dynamic dispatch.</em></li>
	<li>An interface type may be implemented by at most one JSO
	subtype. <em>This ensures that polymorphic dispatch via a
	"SingleJsoImpl" interface can be statically resolved to exactly one
	implementing JSO subtype.</em>
	<ol>
	<li>A <code>JavaScriptObject</code> that implements a
	"SingleJsoImpl" interface may be further extended. The subclasses
	may implement additional "SingleJsoImpl" interfaces. <em>The
	methods on a JSO must be effectively final, so each "SingleJsoImpl"
	method still has a 1:1 mapping to a method defined within a JSO
	subtype.</em></li>
	<li>It is valid for any number of any non-<code>JavaScriptObject</code>
	types to implement a "SingleJsoImpl" interface. <em>There is a
	slight runtime dispatch penalty when a "SingleJsoImpl" interface is
	implemented by both JSO and non-JSO types.</em></li>
	</ol>
	</li>
	<li>No instance methods on JSO classes may override another
	method. <em>This catches accidents where JSO itself did not
	finalize some method from its superclass.</em></li>
	<li>JSO classes cannot have instance fields. <em>The fields
	would have no place to live in web mode. Programmers should instead
	make an explicit wrapper class and put the fields there.</em></li>
	<li>Nested JSO classes must be static. <em>The implicit <code>this</code>
	fields of a non-static inner class has the same problems as an
	explicit field.</em></li>
	<li>"new" operations cannot be used with JSO classes. <em>This
	avoids ever being able to try to instantiate JSO objects using the new
	keyword. New JSO instances can only come from JSNI, as in previous
	versions of GWT.</em></li>
	<li>JSNI methods may not refer to instance methods defined within
	a <code>JavaScriptObject</code>.</li>
	<li>Every JSO class must have precisely one constructor, and it
	must be protected, empty, and no-argument.</li>
	</ol>
	</body>
	</html>