How Beans Are Implemented in Java

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101 How Beans Are Implemented
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Internally, a bean is an instance of a Java class, although in common terminology, the class itself may also be referred to as a bean The most basic kind of bean exposes a number of properties by following a few simple rules regarding method names In general, a bean provides two methods for each property: a method to get the property and one to set the property, corresponding directly to the jsp:getProperty and jsp:setProperty tags Together, these methods are known as accessors Listing 101 shows a very simple bean with two properties Listing 101 A simple bean
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public class SimpleBean { private int age; private String name;
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public int getAge() {return age;} public void setAge(int age) {thisage = age;}
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public String getName() {return name;} public void setName(String name) {thisname = name;} }
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This bean could be used in a JSP just like any other that has appeared throughout this book:
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<jsp:useBean
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<c:out value="${myBeanname}"/>
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In general, for a property named foo of type type, the get method will return an element of type and will be called getFoo() The one exception is that if the type is boolean, the get method may be called isFoo() For example, if it needs to keep track of whether it is ready to perform an action, a bean might have a ready property, and the method could be called isReady() Whether getFoo() or isFoo() is used, it is this method that is called by the jsp:getProperty tag, as well as any tag, such as c:out or c:if, that obtains a value from a bean Similarly, the set method will accept an argument of type, will be called setFoo(), and will be called by jsp:setProperty and c:set tags There is no restriction on the type; it may be something simple, such as an integer or String, or it may be a class or interface type The type may also be an array of another type, in which case the property is called an indexed property In this case, the accessor methods operate on the whole array, and the bean may wish to provide methods to operate on the individual elements, as in Listing 102 Listing 102 A bean with an array property
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public class ArrayBean { private String things[];
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public String[] getThings() {return things;} public void setThings(String things[]) {
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thisthings = things; }
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public String getThings(int i) {return things[i];} public void setThings(int i, String thing) { things[i] = thing; } }
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If an attempt is made to set or get an element with an index larger than the size of the array, the method will throw an ArrayOutOfBounds exception This could, and probably should, be made explicit in the definition of the methods In either case, any calling class should be prepared for this exception and should catch and recover appropriately Technically, there is no reason why trying to get or set an element outside the array could not be trapped and handled by the bean, perhaps by creating a larger array and copying all the existing elements to it The JavaBean specification states that the only way to change the size of an array is to use the array version of the set method, passing in a larger array Programmers can weigh the value of adhering strictly to the standards against the need to catch exceptions elsewhere in their programs Although it is customary to provide both a set and a get method, doing so is not required A property that cannot be set is called a read-only property, and one with no get method is called write-only Read-only properties are fairly common; write-only ones are less so Nothing that has been said so far places any restrictions on what these accessor methods do The preceding examples simply held their properties in private variables, but as long as the naming conventions are maintained, any other Java class, including a servlet or JSP, will be able to discover and access the properties To illustrate this point, Listing 103 shows the DateBean that was used in Listing 32 Listing 103 More complex accessor methods
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import javatext*; import javautil*; import javaioSerializable;
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public class DateBean implements Serializable {
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public DateBean() {}
SimpleDateFormat sdf;
public void setFormat(String format) { sdf = new SimpleDateFormat(format);
public String getCurrentTime() { return sdfformat(new Date()); } }
This example has both a read-only and a write-only property, although there is no fundamental reason why a getFormat() method could not be provided On the other hand, a setDate() method would presumably need to alter time, which will not be possible until Sun comes out with a "Java 2 time traveler's edition" This example also contains an explicit constructor, even though that constructor doesn't do anything A bean is allowed to provide as many different constructors as the programmer wants, but it must have one constructor that takes no arguments