Advanced Java Course Reflection Reflection API What if you want - - PowerPoint PPT Presentation

Advanced Java Course

Reflection

Reflection API

• What if you want to access

information not just about the Object, but about that Object’s Class?

• What if you want to access a method,

field, or constructor whose name you don’t know at compile time?

Example: WebCrawler timer

• To optimize your WebCrawler, first find
• ut how long each big operation is taking.

– Getting content from URLs – Parsing pages – Setting up Database Connection & Statement – Deleting from the Database – Inserting into the Database – Other?

How to time an operation?

• Without reflection:

int startTime = System.currentTimeMillis(); doOperation(...); int endTime = System.currentTimeMillis(); int duration = endTime – startTime;

• Problem:

Code is repeated every place you want to time an

• peration! Not clean at all...

A better way to time an operation

• Object timeMethod(String methodToCall,

Class[] argTypes, Object[] args, String timeToUpdateField) {...}

• timeMethod(“methodName”, new Class[]

{...}, new Object[] {...}, “timeStoringField”);

• No code repetition, less clutter!

Classes in the Reflection API

• Class Class
• Class Constructor
• Class Field
• Class Method

Getting an Object of type Class

• If you have an instance of the class:

– Class classObject = instanceObject.getClass();

• If you know the name of the class at compile time:

– Class classObject = ClassName.class

• If you know the name of the class at run time:

– Class.forName(“...”);

• If you want a primitive type class, and you know the name
• f the wrapper class at compile time;

– Integer.TYPE, Long.TYPE, etc

some methods in the Class class

• getName()
• getSuperClass()
• getComponentType() – [null if !isArray()]
• isArray()
• isInterface()
• isPrimitive()
• isInstance(Object o)
• Object newInstance()

Getting Constructors/Methods/Fields

• ask the Class Object for them

– classObject.getConstructors() – classObject.getConstructor( Class[] argTypes) – classObject.getDeclaredConstructors() – classObject.getDeclaredConstructors( Class[] argTypes)

• Methods and Fields have analogous getters in class

Class.

• you can get an array of all the c/m/f in the class, or just
• ne by name, and if you ask for “Declared” it gives you

the non-public ones as well.

Objects of Type Method

• calling a method dynamically:

– Object methodObject.invoke(Object o, Object[] args);

• getting info about it:

– Class[] getParameterTypes() – Class getReturnType() – Class[] getExceptionTypes()

Objects of type Field

• Object fieldObject.get( Object o );

– also getInt(Object), getBoolean(Object), etc.

• void fieldObject.set( Object o, Object value );

– also setInt(Object o, int i), etc.

• Class fieldObject.getType() [compile time, of

course]

• Fields of type int will have Integer.TYPE as the

result of getType(), and it’s a Class Object.

Objects of type Constructor

• Object constructorObject.newInstance

( Object[] args )

• Class[]

constructorObject.getExceptionTypes()

• Class[]

constructorObject.getParameterTypes()

Member Interface

• Method, Field, and Constructor classes all

extend the Member Interface

• Class getDeclaringClass()
• String getName()
• int getModifiers()

Modifier class (static methods)

• How do you know if something is static, final,

etc.?

• int getModifiers() [from Member Interface]
• Modifer.isStatic(int)
• Modifer.isFinal(int)
• Modifier.isProtected(int)
• etc.

Array class (static methods)

• Object Array.get(Object arrayObject, int i);
• void Array.set(Object arrayObject, int index,

Object value)

• int Array.getLength(Object arrayObject);
• Object newInstance(Class type, int length)
• Object newInstance(Class type, int[] dimensions)

Reflection Group Task #1

• Fill in example from beginning of lecture:

Object timeMethod(String methodToCall, Class[] argTypes, Object[] args, String timeToUpdateField) { ... Please fill this in! ... }

Solution to Reflection Group Task 1

Object timeMethod(String methodToCall, Class[] argTypes, Object[] args, String timeToUpdateField) throws NoSuchMethodException, NoSuchFieldException, InvocationTargetException, IllegalAccessException { Class c = this.getClass(); long startTime = System.currentTimeMillis(); Method m = c.getMethod(methodToCall, argTypes); Object returnValue = m.invoke(this, args); long endTime = System.currentTimeMillis(); Field f = c.getField(timeToUpdateField); f.setLong(this, f.getLong(this) + (endTime - startTime)); return returnValue; }

Reflection Group Task #2

Fill in the class definition to call all getX methods and print their values in the form: ClassName propertyName = value import java.lang.reflect.*; public class PropertyValueFinder { final String GET = "get"; public void printProperties(Object o) { /*fill this in*/ } private Property propertyGottenBy(Method method) { /*fill this in*/} private boolean isGetter(Method method) { /*fill this in*/ } public class Property { String name; Class type; Object value; public Property(String name, Class type) { this.name = name; this.type = type; } public void setValue(Object value) { this.value = value; } public String toString() { return type+" "+name+" = "+value; } } }

Group Task #2 Solutions

private boolean isGetter(Method m) { boolean nameOK = m.getName().startsWith(GET); boolean noArgs = m.getParameterTypes().length == 0; boolean nonVOID = m.getReturnType() != Void.TYPE; return nameOK && noArgs && nonVOID; } private Property propertyGottenBy(Method m) { if (!isGetter(m)) return null; Class propertyType = m.getReturnType(); String propertyName = m.getName(). substring(GET.length()); return new Property(propertyName, propertyType); }

Group Task #2 Solutions

public void printProperties(Object o) { Class oClass = o.getClass(); Method[] methods = oClass.getMethods(); for (int i = 0; i < methods.length; i++) { Method m = methods[i]; if (isGetter(m)) { try { Object value = m.invoke(o, new Object[0]); Property p = propertyGottenBy(m); p.setValue(value); System.out.println(p); } catch (IllegalAccessException e) { throw new RuntimeException(); //programmer error } catch (IllegalArgumentException e) { throw new RuntimeException(); //programmer error } catch (InvocationTargetException e) { throw new RuntimeException(); //programmer error } } } }

Discussion Topic

• Restriction to Serializer project:
• I must promise that the Objects I test your code on will

have a no-args constructor that:

– sets the value of any final variables that are not set in the class definition – does not throw any exceptions.

• If you could add functionality to the Reflection API, how

would you fix this without compromising the integrity of the keyword final?

• Work in teams, and then we’ll exchange ideas.
• Great, now go start a JSR (Java Specification Request)!

– http://www.jcp.org/en/participation/membership