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| <title>Join Points</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="semantics.html" title="Appendix B. Language Semantics"><link rel="previous" href="semantics.html" title="Appendix B. Language Semantics"><link rel="next" href="semantics-pointcuts.html" title="Pointcuts"></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">Join Points</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="semantics.html">Prev</a> </td><th width="60%" align="center">Appendix B. Language Semantics</th><td width="20%" align="right"> <a accesskey="n" href="semantics-pointcuts.html">Next</a></td></tr></table><hr></div><div class="sect1"><a name="semantics-joinPoints"></a><div class="titlepage"><div><h2 class="title" style="clear: both"><a name="semantics-joinPoints"></a>Join Points</h2></div></div><p> | |
| While aspects define types that crosscut, the AspectJ system does not | |
| allow completely arbitrary crosscutting. Rather, aspects define types | |
| that cut across principled points in a program's execution. These | |
| principled points are called join points. | |
| </p><p> | |
| A join point is a well-defined point in the execution of a | |
| program. The join points defined by AspectJ are: | |
| </p><div class="variablelist"><dl><dt><a name="d0e4541"></a><span class="term">Method call</span></dt><dd> | |
| When a method is called, not including super calls of | |
| non-static methods. | |
| </dd><dt><a name="d0e4546"></a><span class="term">Method execution</span></dt><dd> | |
| When the body of code for an actual method executes. | |
| </dd><dt><a name="d0e4551"></a><span class="term">Constructor call</span></dt><dd> | |
| When an object is built and that object's initial constructor is | |
| called (i.e., not for "super" or "this" constructor calls). The | |
| object being constructed is returned at a constructor call join | |
| point, so its return type is considered to be the type of the | |
| object, and the object itself may be accessed with <tt>after | |
| returning</tt> advice. | |
| </dd><dt><a name="d0e4559"></a><span class="term">Constructor execution</span></dt><dd> | |
| When the body of code for an actual constructor executes, after | |
| its this or super constructor call. The object being constructed | |
| is the currently executing object, and so may be accessed with | |
| the <tt>this</tt> pointcut. The constructor execution | |
| join point for a constructor that calls a super constructor also | |
| includes any non-static initializers of enclosing class. No | |
| value is returned from a constructor execution join point, so its | |
| return type is considered to be void. | |
| </dd><dt><a name="d0e4567"></a><span class="term">Static initializer execution</span></dt><dd> | |
| When the static initializer for a class executes. No value is | |
| returned from a static initializer execution join point, so its | |
| return type is considered to be void. | |
| </dd><dt><a name="d0e4572"></a><span class="term">Object pre-initialization</span></dt><dd> | |
| Before the object initialization code for a particular class runs. | |
| This encompasses the time between the start of its first called | |
| constructor and the start of its parent's constructor. Thus, the | |
| execution of these join points encompass the join points of the | |
| evaluation of the arguments of <tt>this()</tt> and | |
| <tt>super()</tt> constructor calls. No value is | |
| returned from an object pre-initialization join point, so its | |
| return type is considered to be void. | |
| </dd><dt><a name="d0e4583"></a><span class="term">Object initialization</span></dt><dd> | |
| When the object initialization code for a particular class runs. | |
| This encompasses the time between the return of its parent's | |
| constructor and the return of its first called constructor. It | |
| includes all the dynamic initializers and constructors used to | |
| create the object. The object being constructed is the currently | |
| executing object, and so may be accessed with the | |
| <tt>this</tt> pointcut. No value is returned from a | |
| constructor execution join point, so its return type is | |
| considered to be void. | |
| </dd><dt><a name="d0e4591"></a><span class="term">Field reference</span></dt><dd> | |
| When a non-constant field is referenced. [Note that references | |
| to constant fields (static final fields bound to a constant | |
| string object or primitive value) are not join points, since Java | |
| requires them to be inlined.] | |
| </dd><dt><a name="d0e4596"></a><span class="term">Field set</span></dt><dd> | |
| When a field is assigned to. | |
| Field set join points are considered to have one argument, | |
| the value the field is being set to. | |
| No value is returned from a field set join point, so | |
| its return type is considered to be void. | |
| [Note that the initializations of constant fields (static | |
| final fields where the initializer is a constant string object or | |
| primitive value) are not join points, since Java requires their | |
| references to be inlined.] | |
| </dd><dt><a name="d0e4601"></a><span class="term">Handler execution</span></dt><dd> | |
| When an exception handler executes. | |
| Handler execution join points are considered to have one argument, | |
| the exception being handled. | |
| No value is returned from a field set join point, so | |
| its return type is considered to be void. | |
| </dd><dt><a name="d0e4606"></a><span class="term">Advice execution</span></dt><dd> | |
| When the body of code for a piece of advice executes. | |
| </dd></dl></div><p> | |
| Each join point potentially has three pieces of state associated | |
| with it: the currently executing object, the target object, and | |
| an object array of arguments. These are exposed by the three | |
| state-exposing pointcuts, <tt>this</tt>, | |
| <tt>target</tt>, and <tt>args</tt>, | |
| respectively. | |
| </p><p> | |
| Informally, the currently executing object is the object that a | |
| <tt>this</tt> expression would pick out at the join | |
| point. The target object is where control or attention is | |
| transferred to by the join point. The arguments are those | |
| values passed for that transfer of control or attention. | |
| </p><div class="informaltable" id="d0e4627"><a name="d0e4627"></a><table border="1"><colgroup><col><col><col><col></colgroup><thead valign="top"><tr><th><span class="bold"><b>Join Point</b></span></th><th><span class="bold"><b>Current Object</b></span></th><th><span class="bold"><b>Target Object</b></span></th><th><span class="bold"><b>Arguments</b></span></th></tr></thead><tbody><tr><td>Method Call</td><td>executing object*</td><td>target object**</td><td>method arguments</td></tr><tr><td>Method Execution</td><td>executing object*</td><td>executing object*</td><td>method arguments</td></tr><tr><td>Constructor Call</td><td>executing object*</td><td>None</td><td>constructor arguments</td></tr><tr><td>Constructor Execution</td><td>executing object</td><td>executing object</td><td>constructor arguments</td></tr><tr><td>Static initializer execution</td><td>None</td><td>None</td><td>None</td></tr><tr><td>Object pre-initialization</td><td>None</td><td>None</td><td>constructor arguments</td></tr><tr><td>Object initialization</td><td>executing object</td><td>executing object</td><td>constructor arguments</td></tr><tr><td>Field reference</td><td>executing object*</td><td>target object**</td><td>None</td></tr><tr><td>Field assignment</td><td>executing object*</td><td>target object**</td><td>assigned value</td></tr><tr><td>Handler execution</td><td>executing object*</td><td>executing object*</td><td>caught exception</td></tr><tr><td>Advice execution</td><td>executing aspect</td><td>executing aspect</td><td>advice arguments</td></tr></tbody></table></div><p>* There is no executing object in static contexts such as | |
| static method bodies or static initializers. | |
| </p><p>** There is no target object for join points associated | |
| with static methods or fields. | |
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