initialization join points in Java

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initialization join points
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In this code snippet, if the first constructor is called, the object initialization join point encompasses the assignment to the _isOverdraft instance member and not the super(). If the second constructor is invoked, the call to this() and the assignment in the first constructor form the join point. Object pre-initialization join points The object pre-initialization join point is rarely used. It encompasses the passage from the constructor that was called first to the beginning of its parent constructor. Practically, it encompasses calls made while forming arguments to the super() call in the constructor. This code shows the object pre-initialization join points:
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public class SavingsAccount extends Account { ...
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Introducing AspectJ
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public SavingsAccount(int accountNumber) { super(accountNumber, AccountManager.internalId(accountNumber) ); _type = AccountConstants.SAVINGS; } ... }
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Object preinitialization join point
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In this code snippet, the object pre-initialization encompasses a call to the AccountManager.internalId(accountNumber) method only and not the entire super() call. Advice execution join points The advice execution join point is a recent addition to the AspectJ language. It encompasses the execution of every advice in the system. This join point s usage is not yet fully established. Such join points may be used to advise an advice for purposes such as profiling the advice itself. This code shows the advice execution join point:
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public aspect MannersAspect { before() : deliverMessage() { System.out.print("Hello! "); } } public aspect LoggingAspect { after() : loggedOperations() { ... _logger.log(...); ... } }
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In this code snippet, the advice execution join points encompasses the before and after advice in MannersAspect and LoggingAspect. 2.4.2 Join point demonstration example Let s create an example to help solidify our understanding of important join points by printing out the join points that are encountered as the code is executed. First, we will set up a simple class structure with an Account class and its subclass SavingsAccount. This inheritance helps us understand the join points occurring in the method and constructor invocation between the base and
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The join point model
derived classes. In this example, we need to jump ahead a little and use some simple AspectJ constructs pointcuts and advice which we will explain in detail in chapter 3. The Account class in listing 2.5 represents a simplified version of a bank account. It contains methods for performing the debit and credit operations as well as getting and setting the account balance.
Listing 2.5 Account.java
public abstract class Account { private float _balance; private int _accountNumber; public Account(int accountNumber) { _accountNumber = accountNumber; } public void credit(float amount) { setBalance(getBalance() + amount); } public void debit(float amount) throws InsufficientBalanceException { float balance = getBalance(); if (balance < amount) { throw new InsufficientBalanceException( "Total balance not sufficient"); } else { setBalance(balance - amount); } } public float getBalance() { return _balance; } public void setBalance(float balance) { _balance = balance; } }
The debit() method of the Account class declares that it may throw InsufficientBalanceException when the account balance is not sufficient to perform the operation. Listing 2.6 shows the implementation of this exception.
Introducing AspectJ
Listing 2.6 InsufficientBalanceException.java
class InsufficientBalanceException extends Exception { public InsufficientBalanceException(String message) { super(message); } }
The SavingsAccount class is a specialization of the Account class that represents the savings account. In our example in listing 2.7, it serves to show that the join point for classes is connected with the inheritance relationship.
Listing 2.7 SavingsAccount.java
public class SavingsAccount extends Account { public SavingsAccount(int accountNumber) { super(accountNumber); } }
Listing 2.8 is a simple test program that will cause the execution of the join points.
Listing 2.8 Test.java
public class Test { public static void main(String[] args) throws InsufficientBalanceException { SavingsAccount account = new SavingsAccount(12456); account.credit(100); account.debit(50); } }
Next, let s write a simple tracing aspect, shown in listing 2.9, that prints the information for all join points as the code executes. Since we have not officially discussed the details of the pointcut, we will use only a within() pointcut along with a negation operator to capture all the join points occurring outside the aspect itself. (You ll learn more about the within() pointcut in the next chapter.) The before and after advice prints the information about the join points captured by the tracePoints() pointcut. We also use a special variable thisJoinPoint that is available in each advice body; thisJoinPoint is a special
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