Defensive Programming. Ric Glassey

Transcription

1 Defensive Programming Ric Glassey

2 Outline Defensive Programming Aim: Develop the programming skills to anticipate problems beyond control that may occur at runtime Responsibility Exception handling

3 Responsibility

4 Client/Server Interaction Client Server + int fieldone + String fieldtwo - operationone(int x) - operationtwo(int y, String z)

5 Spectrum of Trust Trust the client Never trust the client

6 Developer questions How much checking should a server s methods perform on client requests? How should a server report errors to its clients? How can a client anticipate failure of a request to a server? How should a client deal with failure of a request?

7 Return Value vs Exception There are two strategies that are useful to notify clients that something has went wrong Return a boolean False of Null object reference Create and throw a special Exception object and use it to communicate the problem details

8 Exception Handling

9 Exceptions In response to an unexpected event When error occurs, an object (the Exception) is created and handed to runtime system The exception contains information, including its type and state of the program Referred to as throwing an exception

10 Tracing the call stack Method where error occurred Method Call Method without exception handler Method Call Method with an exception handler Method Call Launch Application main The Call Stack of an Application

11 Exception Handling After an exception is thrown, the runtime will try to locate the relevant exception handler Runtime searches back through the call stack and will stop at the first relevant exception handler

12 Re-Tracing the call stack Throws exception Method where error occurred Looking for exception handler Method Call Forwards exception Method without exception handler Looking for exception handler Method Call Catches other exception type Method with an exception handler Looking for exception handler Method Call Abort program; print stack trace main The Call Stack of an Application

13 Catch or Specify Requirement for code that might throw exception: Possess a try statement to catch exception Method specifies that the exception can be thrown using the throws clause

14 Kinds of Exceptions Checked Exception Application should anticipate and recover from e.g. java.io.filenotfoundexception Error* Circumstances external to the application e.g. Hardware failure Runtime Exception* Internal to the application, typically bugs e.g. NullPointerException (n.b. can be caught, but better to abort and fix * does not require catch or specify requirement

15 For the lazy programmer Both Error and RuntimeExceptions are unchecked exceptions Programmers can avoid the catch or specify requirement by extending their exception classes from Error or RuntimeExceptions Silences the compiler :-) MyException (Checked) Exception Throwable RuntimeException (Unchecked) MyRuntimeException (Unchecked) Error (Unchecked)

16 Catching and Handling Exception handling components try catch finally try-with-resource (from Java 7)

17 Using Try-Catch Blocks

18 import java.io.*; import java.util.list; import java.util.arraylist; IndexOutOfBoundsException - UncheckedException IOException - Checked public class ListOfNumbers { private List<Integer> list; private static final int SIZE = 10; public ListOfNumbers () { list = new ArrayList<Integer>(SIZE); for (int i = 0; i < SIZE; i++) { list.add(new Integer(i)); public void writelist() { PrintWriter out = new PrintWriter(new FileWriter("OutFile.txt")); for (int i = 0; i < SIZE; i++) { out.println("value at: " + i + " = " + list.get(i)); out.close(); Locate two potential exceptions

19 Step 1: Add Try block private List<Integer> list; private static final int SIZE = 10; public void writelist() { PrintWriter out = null; try { // Exception thrown somewhere within this block System.out.println("Entered try statement"); out = new PrintWriter(new FileWriter("OutFile.txt")); for (int i = 0; i < SIZE; i++) { out.println("value at: " + i + " = " + list.get(i)); // End of try block catch and finally blocks

20 Step 2: Add Catch block try { System.out.println("Entered try statement"); out = new PrintWriter(new FileWriter("OutFile.txt")); for (int i = 0; i < SIZE; i++) { out.println("value at: " + i + " = " + list.get(i)); // Add one or more catch blocks after try block catch (IndexOutOfBoundsException e) { System.err.println("IndexOutOfBoundsException: " + e.getmessage()); catch (IOException e) { System.err.println("Caught IOException: " + e.getmessage());

21 Step 3: Add optional Finally block finally { if (out!= null) { System.out.println("Closing PrintWriter"); out.close(); else { System.out.println("PrintWriter not open"); Finally block is always executed Useful place to perform cleanup work after success or fail Typical usage is to release resources by calling close() Avoids resource leaks

22 public void writelist() { PrintWriter out = new PrintWriter(new FileWriter("OutFile.txt")); for (int i = 0; i < SIZE; i++) { out.println("value at: " + i + " = " + list.get(i)); out.close(); Initial form characters

23 public void writelist() { PrintWriter out = null; try { out = new PrintWriter(new FileWriter("OutFile.txt")); for (int i = 0; i < SIZE; i++) { out.println("value at: " + i + " = " + list.get(i)); catch (IndexOutOfBoundsException e) { System.err.println("Caught IndexOutOfBoundsException: " + e.getmessage()); catch (IOException e) { System.err.println("Caught IOException: " + e.getmessage()); finally { if (out!= null) { System.out.println("Closing PrintWriter"); out.close(); else { System.out.println("PrintWriter not open"); Final form characters

24 Try-with-resource alternative static String readfirstlinefromfile(string path) throws IOException { try (BufferedReader br = new BufferedReader(new FileReader(path))) { return br.readline(); Try statement that declares one or more resources Resources are objects that must be released after use Requires the object to implement java.lang.autocloseable

25 Throwing Exceptions

26 Using throws clause The current method may not always be the appropriate place to deal with an exception Instead, exception handling can be located elsewhere, and exceptions forwarded up the call stack public void writelist() throws IOException, IndexOutOfBoundsException { PrintWriter out = new PrintWriter(new FileWriter("OutFile.txt")); for (int i = 0; i < SIZE; i++) { out.println("value at: " + i + " = " + list.get(i)); out.close(); Do we need both to be declared?

27 Using throw statement Exceptions can be generated from any point in a program Simply throw new ExceptionType; public Object pop() { Object obj; if (size == 0) { throw new EmptyStackException(); obj = objectat(size - 1); setobjectat(size - 1, null); size--; return obj;

28 Best Practice with Exceptions

29 Effective Java Chp 9. Use exceptions only for exceptional conditions // Horrible abuse of exceptions. Don't ever do this! try { int i = 0; while(true) range[i++].climb(); catch(arrayindexoutofboundsexception e) {

30 Effective Java Chp 9. Use checked expressions for recoverable conditions and runtime exceptions for programming errors e.g. File not found vs array indexing problem

31 Effective Java Chp 9. Avoid unnecessary use of checked exceptions Creates a difficult to use API Favour the standard exceptions: IllegalArgumentException, IllegalStateException NullPointerException, IndexOutOfBoundsException ConcurrentModificationException UnsupportedOperationException

32 Effective Java Chp 9. Document all exceptions thrown by methods Include failure-capture information in detail messages Don t ignore exceptions

33 Readings Objects First with Java, 5th Ed. **required** Chapter 12: Handling Errors KTH:one_search:KTH_LMSMILL Effective Java, 2nd Ed. Chapter 9: Exceptions KTH:one_search:KTH_SFX The Java Tutorial: Exceptions