Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

22.7 Java-to-JavaScript Data Conversion

In the last two sections, we discussed
the rules by which values are converted when JavaScript reads and
writes Java fields and invokes Java methods. Those rules explained
how the JavaScript JavaObject, JavaArray, and JavaClass objects
convert data; they apply only to the case of JavaScript manipulating
Java. When Java manipulates JavaScript, the conversion is performed
by the Java JSObject class, and the conversion
rules are different. Figure 22-4 and Figure 22-5 illustrate these conversions.

Figure 22-4. Data conversions performed when Java writes JavaScript values

Figure 22-5. Data conversions performed when Java reads JavaScript values

The point to remember when studying these figures is that Java can
interact with JavaScript only through the API provided by the
JSObject
class. Because Java is a strongly typed language, the methods defined
by this class can work only with Java objects, not with primitive
values. For example, when you read the value of a JavaScript number,
the getMember( ) method returns a
java.lang.Double object, rather than a primitive
double value.

When writing JavaScript
functions that are invoked from Java,
bear in mind that the arguments passed by Java are either JavaScript
objects from unwrapped Java JSObjects, or JavaObjects. LiveConnect
simply does not allow Java to pass primitive values as method
arguments. As we saw earlier in this chapter, JavaObject objects
behave somewhat differently than other objects. For example, an
instance of

java.lang.Double behaves differently
than a primitive JavaScript number or even a JavaScript Number
object. The same caution applies when you are working with JavaScript
properties that have their values set by Java.

One way to avoid the whole issue of
data conversion is to use the eval( ) method of
the JSObject class whenever your Java code wants to communicate with
JavaScript. In order to do this, your Java code must convert all
method arguments or property values to string form. Then, the string
to be evaluated can be passed unchanged to JavaScript, which can
convert the string form of the data to the appropriate JavaScript
data values.