java qr code reader Common logging idioms in Java

Drawing QR-Code in Java Common logging idioms

5.5 Common logging idioms
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Idioms are solutions to simple, recurring problems. While developing a logging solution, you will encounter a few common issues, such as logging the operation s context, beautifying the output for human consumption, considering the precedence for other aspects, and changing the logging toolkit. In this section, we discuss common idioms that should guide you in your attempts to introduce logging in your system.
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Monitoring techniques
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5.5.1 Logging the method parameters Often, you not only want to log the method calls but also the invoked object and the method parameters. Implementing this requirement is easily accomplished by using the thisJoinPoint reference. In each advice body, a special thisJoinPoint object is available that contains the information about the captured join point and its associated context. The aspect in listing 5.17 modifies the TraceAspectV1 s before advice to also log the method parameters.
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Listing 5.17 TraceAspectV1.java: modified to log method parameters
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import org.aspectj.lang.*;
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Augment public aspect TraceAspectV1 { pointcut pointcut traceMethods() that avoids : (execution(* *.*(..)) infinite || execution(*.new(..))) && !within(TraceAspectV1); recursion
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before() : traceMethods()&& !execution(String *.toString()){ Signature sig = thisJoinPointStaticPart.getSignature(); System.err.println("Entering [" + sig.getDeclaringType().getName() + "." + sig.getName() + "]" + createParameterMessage(thisJoinPoint)); }
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Formatting of the log message
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private String createParameterMessage(JoinPoint joinPoint) { StringBuffer paramBuffer = new StringBuffer("\n\t[This: "); paramBuffer.append(joinPoint.getThis()); Object[] arguments = joinPoint.getArgs(); paramBuffer.append("]\n\t[Args: ("); for (int length = arguments.length, i = 0; i < length; ++i) { Object argument = arguments[i]; paramBuffer.append(argument); if (i != length-1) { paramBuffer.append(','); } } paramBuffer.append(")]"); return paramBuffer.toString(); } }
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Common logging idioms
We use the !execution(String *.toString())pointcut to avoid the recursion that will be caused by the execution of the toString() methods. Without this pointcut, the logger will prepare the parameter string in createParameterMessage() when it calls toString() for each object. However, when toString() executes, it first attempts to log the operation, and the logger will prepare a parameter string for it again when it calls toString() on the same object, and so on, causing an infinite recursion. By avoiding the join points for toString() execution, we avoid infinite recursion, leading to a stack overflow. Note that the !within(TraceAspectV1) pointcut is not sufficient here because it will only capture the calls to toString() methods; the execution of the methods will still be advised. The createParameterMessage() helper method returns a formatted string containing the object and arguments. Now when we compile the classes in our shopping cart example with this aspect and execute the Test class, the output of logging includes the invoked object and the method parameters, similar to this output:
> ajc *.java > java Test Entering [Test.main] [This: null] [Args: ([Ljava.lang.String;@1eed786)] Entering [Inventory.<init>] [This: Inventory@e48e1b] [Args: ()] Entering [Item.<init>] [This: Item: null] [Args: (1,30.0)] Entering [Item.<init>] [This: Item: null] [Args: (2,31.0)] Entering [Item.<init>] [This: Item: null] [Args: (3,32.0)] Entering [Inventory.addItem] [This: Inventory@e48e1b] [Args: (Item: 1)] Entering [Inventory.addItem] [This: Inventory@e48e1b] [Args: (Item: 2)] Entering [Inventory.addItem] [This: Inventory@e48e1b] [Args: (Item: 3)] Entering [ShoppingCart.<init>] [This: ShoppingCart@30c221] [Args: ()]
Monitoring techniques
Entering [ShoppingCartOperator.addShoppingCartItem] [This: null] [Args: (ShoppingCart@30c221,Inventory@e48e1b,Item: 1)] Entering [Inventory.removeItem] [This: Inventory@e48e1b] [Args: (Item: 1)] Entering [ShoppingCart.addItem] [This: ShoppingCart@30c221] [Args: (Item: 1)] Entering [ShoppingCartOperator.addShoppingCartItem] [This: null] [Args: (ShoppingCart@30c221,Inventory@e48e1b,Item: 2)] Entering [Inventory.removeItem] [This: Inventory@e48e1b] [Args: (Item: 2)] Entering [ShoppingCart.addItem] [This: ShoppingCart@30c221] [Args: (Item: 2)
NOTE TO STANDARD JAVA LOGGING KIT USERS
You cannot easily log the method arguments if you use the version of the Logger.logp() method that takes an arbitrary Object array as an argument. The reason is that the method requires that you supply the message in the format specified by the MessageFormat class. You can implement this if you know the method argument types, position, and count. However, this is not very practical for a general-purpose logging scheme. The solution, therefore, is to build your own message that contains the argument information before you pass it to the logger object.
5.5.2 Indenting the log statements With nested operations, it is desirable that the logging mimic the operation depth. For stream output, this usually means indenting the log messages based on their depth in the transaction. Without such information, the output is hard to decipher. The basic idea behind implementing the indentation functionality is simple: keep a state corresponding to the call depth of the currently executing operation and prefix each log message with spaces proportionate to the call depth. The concrete implementation of this idea can be implemented in various ways. You have already seen one way in chapter 2, listing 2.9. In this section, we examine another implementation that offers the advantage of reusability. The abstract aspect shown in listing 5.18 encapsulates the indentation functionality. By simply extending this aspect, a logging aspect can introduce the indentation effect.
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