| <html><head> |
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| <title>Reusable Aspects</title><link rel="stylesheet" href="aspectj-docs.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.44"><link rel="home" href="index.html" title="The AspectJTM Programming Guide"><link rel="up" href="examples.html" title="Chapter 3. Examples"><link rel="previous" href="examples-production.html" title="Production Aspects"><link rel="next" href="idioms.html" title="Chapter 4. Idioms"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Reusable Aspects</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="examples-production.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Examples</th><td width="20%" align="right"> <a accesskey="n" href="idioms.html">Next</a></td></tr></table><hr></div><div class="sect1"><a name="examples-reusable"></a><div class="titlepage"><div><h2 class="title" style="clear: both"><a name="examples-reusable"></a>Reusable Aspects</h2></div></div><div class="sect2"><a name="tracing-using-aspects-revisited"></a><div class="titlepage"><div><h3 class="title"><a name="tracing-using-aspects-revisited"></a>Tracing using Aspects, Revisited</h3></div></div><p> |
| (The code for this example is in |
| <tt><i><tt>InstallDir</tt></i>/examples/tracing</tt>.) |
| </p><div class="sect3"><a name="d0e3391"></a><div class="titlepage"><div><h4 class="title"><a name="d0e3391"></a>Tracing—Version 3</h4></div></div><p> |
| One advantage of not exposing the methods traceEntry and |
| traceExit as public operations is that we can easily change their |
| interface without any dramatic consequences in the rest of the |
| code. |
| </p><p> |
| Consider, again, the program without AspectJ. Suppose, for |
| example, that at some point later the requirements for tracing |
| change, stating that the trace messages should always include the |
| string representation of the object whose methods are being |
| traced. This can be achieved in at least two ways. One way is |
| keep the interface of the methods <tt>traceEntry</tt> |
| and <tt>traceExit</tt> as it was before, |
| </p><pre class="programlisting"> |
| public static void traceEntry(String str); |
| public static void traceExit(String str); |
| </pre><p> |
| In this case, the caller is responsible for ensuring that the |
| string representation of the object is part of the string given |
| as argument. So, calls must look like: |
| </p><pre class="programlisting"> |
| Trace.traceEntry("Square.distance in " + toString()); |
| </pre><p> |
| Another way is to enforce the requirement with a second argument |
| in the trace operations, e.g. |
| </p><pre class="programlisting"> |
| public static void traceEntry(String str, Object obj); |
| public static void traceExit(String str, Object obj); |
| </pre><p> |
| In this case, the caller is still responsible for sending the |
| right object, but at least there is some guarantees that some |
| object will be passed. The calls will look like: |
| </p><pre class="programlisting"> |
| Trace.traceEntry("Square.distance", this); |
| </pre><p> |
| In either case, this change to the requirements of tracing will |
| have dramatic consequences in the rest of the code -- every call |
| to the trace operations traceEntry and traceExit must be changed! |
| </p><p> |
| Here's another advantage of doing tracing with an aspect. We've |
| already seen that in version 2 <tt>traceEntry</tt> and |
| <tt>traceExit</tt> are not publicly exposed. So |
| changing their interfaces, or the way they are used, has only a |
| small effect inside the <tt>Trace</tt> |
| class. Here's a partial view at the implementation of |
| <tt>Trace</tt>, version 3. The differences with |
| respect to version 2 are stressed in the comments: |
| </p><pre class="programlisting"> |
| abstract aspect Trace { |
| |
| public static int TRACELEVEL = 0; |
| protected static PrintStream stream = null; |
| protected static int callDepth = 0; |
| |
| public static void initStream(PrintStream s) { |
| stream = s; |
| } |
| |
| protected static void traceEntry(String str, Object o) { |
| if (TRACELEVEL == 0) return; |
| if (TRACELEVEL == 2) callDepth++; |
| printEntering(str + ": " + o.toString()); |
| } |
| |
| protected static void traceExit(String str, Object o) { |
| if (TRACELEVEL == 0) return; |
| printExiting(str + ": " + o.toString()); |
| if (TRACELEVEL == 2) callDepth--; |
| } |
| |
| private static void printEntering(String str) { |
| printIndent(); |
| stream.println("Entering " + str); |
| } |
| |
| private static void printExiting(String str) { |
| printIndent(); |
| stream.println("Exiting " + str); |
| } |
| |
| private static void printIndent() { |
| for (int i = 0; i < callDepth; i++) |
| stream.print(" "); |
| } |
| |
| abstract pointcut myClass(Object obj); |
| |
| pointcut myConstructor(Object obj): myClass(obj) && execution(new(..)); |
| pointcut myMethod(Object obj): myClass(obj) && |
| execution(* *(..)) && !execution(String toString()); |
| |
| before(Object obj): myConstructor(obj) { |
| traceEntry("" + thisJoinPointStaticPart.getSignature(), obj); |
| } |
| after(Object obj): myConstructor(obj) { |
| traceExit("" + thisJoinPointStaticPart.getSignature(), obj); |
| } |
| |
| before(Object obj): myMethod(obj) { |
| traceEntry("" + thisJoinPointStaticPart.getSignature(), obj); |
| } |
| after(Object obj): myMethod(obj) { |
| traceExit("" + thisJoinPointStaticPart.getSignature(), obj); |
| } |
| } |
| </pre><p> |
| As you can see, we decided to apply the first design by preserving |
| the interface of the methods <tt>traceEntry</tt> and |
| <tt>traceExit</tt>. But it doesn't matter—we could |
| as easily have applied the second design (the code in the directory |
| <tt>examples/tracing/version3</tt> has the second |
| design). The point is that the effects of this change in the |
| tracing requirements are limited to the |
| <tt>Trace</tt> aspect class. |
| </p><p> |
| One implementation change worth noticing is the specification of |
| the pointcuts. They now expose the object. To maintain full |
| consistency with the behavior of version 2, we should have included |
| tracing for static methods, by defining another pointcut for static |
| methods and advising it. We leave that as an exercise. |
| </p><p> |
| Moreover, we had to exclude the execution join point of the method |
| <tt>toString</tt> from the <tt>methods</tt> |
| pointcut. The problem here is that <tt>toString</tt> is |
| being called from inside the advice. Therefore if we trace it, we |
| will end up in an infinite recursion of calls. This is a subtle |
| point, and one that you must be aware when writing advice. If the |
| advice calls back to the objects, there is always the possibility |
| of recursion. Keep that in mind! |
| </p><p> |
| In fact, esimply excluding the execution join point may not be |
| enough, if there are calls to other traced methods within it -- in |
| which case, the restriction should be |
| </p><pre class="programlisting"> |
| && !cflow(execution(String toString())) |
| </pre><p> |
| excluding both the execution of toString methods and all join |
| points under that execution. |
| </p><p> |
| In summary, to implement the change in the tracing requirements we |
| had to make a couple of changes in the implementation of the |
| <tt>Trace</tt> aspect class, including changing the |
| specification of the pointcuts. That's only natural. But the |
| implementation changes were limited to this aspect. Without |
| aspects, we would have to change the implementation of every |
| application class. |
| </p><p> |
| Finally, to run this version of tracing, go to the directory |
| <tt>examples</tt> and type: |
| </p><pre class="programlisting"> |
| ajc -argfile tracing/tracev3.lst |
| </pre><p> |
| The file tracev3.lst lists the application classes as well as this |
| version of the files <tt>Trace.java</tt> and |
| <tt>TraceMyClasses.java</tt>. To run the program, type |
| </p><pre class="programlisting"> |
| java tracing.version3.TraceMyClasses |
| </pre><p>The output should be:</p><pre class="programlisting"> |
| --> tracing.TwoDShape(double, double) |
| <-- tracing.TwoDShape(double, double) |
| --> tracing.Circle(double, double, double) |
| <-- tracing.Circle(double, double, double) |
| --> tracing.TwoDShape(double, double) |
| <-- tracing.TwoDShape(double, double) |
| --> tracing.Circle(double, double, double) |
| <-- tracing.Circle(double, double, double) |
| --> tracing.Circle(double) |
| <-- tracing.Circle(double) |
| --> tracing.TwoDShape(double, double) |
| <-- tracing.TwoDShape(double, double) |
| --> tracing.Square(double, double, double) |
| <-- tracing.Square(double, double, double) |
| --> tracing.Square(double, double) |
| <-- tracing.Square(double, double) |
| --> double tracing.Circle.perimeter() |
| <-- double tracing.Circle.perimeter() |
| c1.perimeter() = 12.566370614359172 |
| --> double tracing.Circle.area() |
| <-- double tracing.Circle.area() |
| c1.area() = 12.566370614359172 |
| --> double tracing.Square.perimeter() |
| <-- double tracing.Square.perimeter() |
| s1.perimeter() = 4.0 |
| --> double tracing.Square.area() |
| <-- double tracing.Square.area() |
| s1.area() = 1.0 |
| --> double tracing.TwoDShape.distance(TwoDShape) |
| --> double tracing.TwoDShape.getX() |
| <-- double tracing.TwoDShape.getX() |
| --> double tracing.TwoDShape.getY() |
| <-- double tracing.TwoDShape.getY() |
| <-- double tracing.TwoDShape.distance(TwoDShape) |
| c2.distance(c1) = 4.242640687119285 |
| --> double tracing.TwoDShape.distance(TwoDShape) |
| --> double tracing.TwoDShape.getX() |
| <-- double tracing.TwoDShape.getX() |
| --> double tracing.TwoDShape.getY() |
| <-- double tracing.TwoDShape.getY() |
| <-- double tracing.TwoDShape.distance(TwoDShape) |
| s1.distance(c1) = 2.23606797749979 |
| --> String tracing.Square.toString() |
| --> String tracing.TwoDShape.toString() |
| <-- String tracing.TwoDShape.toString() |
| <-- String tracing.Square.toString() |
| s1.toString(): Square side = 1.0 @ (1.0, 2.0) |
| </pre></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="examples-production.html">Prev</a> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right"> <a accesskey="n" href="idioms.html">Next</a></td></tr><tr><td width="40%" align="left">Production Aspects </td><td width="20%" align="center"><a accesskey="u" href="examples.html">Up</a></td><td width="40%" align="right"> Chapter 4. Idioms</td></tr></table></div></body></html> |
