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<chapter id="jpsigs" xreflabel="Join Point Signatures">
<title>Join Point Signatures</title>
<para>
Many of the extensions to the AspectJ language to address the new features of
Java 5 are derived from a simple set of principles for join point
matching. In this section, we outline these principles as a foundation
for understanding the matching rules in the presence of annotations,
generics, covariance, varargs, and autoboxing.
</para>
<sect1 id="join-point-matching">
<title>Join Point Matching</title>
<para>AspectJ supports 11 different kinds of join points. These are
the <literal>method call, method execution, constructor call,
constructor execution, field get, field set, pre-initialization,
initialization, static initialization, handler,</literal> and
<literal>advice execution</literal> join points.</para>
<para>The <emphasis>kinded</emphasis> pointcut designators match
based on the kind of a join point. These are the <literal>call,
execution, get, set, preinitialization, initialization,
staticinitialization, handler,</literal> and <literal>adviceexecution</literal>
designators.</para>
<para>A kinded pointcut is written using patterns, some of which
match based on <emphasis>signature</emphasis>, and some of which
match based on <emphasis>modifiers</emphasis>. For example, in
the <literal>call</literal> pointcut designator:</para>
<programlisting><![CDATA[
call(ModifierPattern TypePattern TypePattern.IdPattern(TypePatternList) ThrowsPattern)
]]></programlisting>
<para>the modifiers matching patterns are <literal>ModifierPattern</literal>
and <literal>ThrowsPattern</literal>, and the signature matching patterns
are <literal>TypePattern TypePattern.IdPattern(TypePatternList)</literal>.
</para>
<para>
A join point has potentially multiple signatures, but only one set of
modifiers. <emphasis>A kinded primitive pointcut matches a particular join point
if and only if</emphasis>:
</para>
<orderedlist>
<listitem>They are of the same kind</listitem>
<listitem>The signature pattern (exactly) matches at least one
signature of the join point</listitem>
<listitem>The modifiers pattern matches the modifiers of the
subject of the join point</listitem>
</orderedlist>
<para>These rules make it very easily to quickly determine whether a
given pointcut matches a given join point. In the next two sections,
we describe what the signature(s) of a join point are, and what the
subjects of join points are.</para>
</sect1>
<sect1 id="join-point-signatures">
<title>Join Point Signatures</title>
<para>Call, execution, get, and set join points may potentially have multiple
signatures. All other join points have exactly one signature. The
following table summarizes the constituent parts of a join point
signature for the different kinds of join point.</para>
<informaltable>
<tgroup cols="7">
<thead>
<row>
<entry>Join Point Kind</entry>
<entry>Return Type</entry>
<entry>Declaring Type</entry>
<entry>Id</entry>
<entry>Parameter Types</entry>
<entry>Field Type</entry>
<entry>Exception Type</entry>
</row>
</thead>
<tbody>
<row>
<entry>Method call</entry>
<entry>+</entry>
<entry>+</entry>
<entry>+</entry>
<entry>+</entry>
<entry></entry>
<entry></entry>
</row>
<row>
<entry>Method execution</entry>
<entry>+</entry>
<entry>+</entry>
<entry>+</entry>
<entry>+</entry>
<entry></entry>
<entry></entry>
</row>
<row>
<entry>Constructor call</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
<entry></entry>
</row>
<row>
<entry>Constructor execution</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
<entry></entry>
</row>
<row>
<entry>Field get</entry>
<entry></entry>
<entry>+</entry>
<entry>+</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
</row>
<row>
<entry>Field set</entry>
<entry></entry>
<entry>+</entry>
<entry>+</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
</row>
<row>
<entry>Pre-initialization</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
<entry></entry>
</row>
<row>
<entry>Initialization</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
<entry></entry>
</row>
<row>
<entry>Static initialization</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
<entry></entry>
<entry></entry>
<entry></entry>
</row>
<row>
<entry>Handler</entry>
<entry></entry>
<entry></entry>
<entry></entry>
<entry></entry>
<entry></entry>
<entry>+</entry>
</row>
<row>
<entry>Advice execution</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
<entry>+</entry>
<entry></entry>
<entry></entry>
</row>
</tbody>
</tgroup>
</informaltable>
<para>Note that whilst an advice execution join point has a
signature comprising the declaring type of the advice and the
advice parameter types, the <literal>adviceexecution</literal>
pointcut designator does not support matching based on this
signature.</para>
<para>The signatures for most of the join point kinds should be
self-explanatory, except for field get and set, and method call and execution
join points, which can have multiple signatures. Each signature of
a method call or execution join point has the same id and parameter
types, but the declaring type and return type (with covariance) may vary.
Each signature of a field get or set join point has the same id and field
type, but the declaring type may vary.
</para>
<para>The following sections examine signatures for these join points
in more detail.</para>
<sect2 id="method-call-join-point-signatures" xreflabel="method-call-join-point-signatures">
<title>Method call join point signatures</title>
<para>
For a call join point where a call is made to a method
<literal>m(parameter_types)</literal> on a target type <literal>T</literal> (where
<literal>T</literal> is the static type of the target):
</para>
<programlisting><![CDATA[
T t = new T();
t.m("hello"); <= call join point occurs when this line is executed
]]></programlisting>
<para>
Then the signature <literal>R(T) T.m(parameter_types)</literal> is a signature
of the call join point, where <literal>R(T)</literal> is the return
type of <literal>m</literal> in <literal>T</literal>, and
<literal>parameter_types</literal> are the parameter types of
<literal>m</literal>. If <literal>T</literal> itself does not
declare a definition of <literal>m(parameter_types)</literal>, then
<literal>R(T)</literal> is the return type in the definition of
<literal>m</literal> that <literal>T</literal> inherits. Given the
call above, and the definition of <literal>T.m</literal>:
</para>
<programlisting><![CDATA[
interface Q {
R m(String s);
}
class P implements Q {
R m(String s) {...}
}
class S extends P {
R' m(String s) {...}
}
class T extends S {}
]]></programlisting>
<para>Then <literal>R' T.m(String)</literal> is a signature of the
call join point for <literal>t.m("hello")</literal>.</para>
<para>
For each ancestor (super-type) <literal>A</literal> of <literal>T</literal>,
if <literal>m(parameter_types)</literal> is defined for that super-type, then
<literal>R(A) A.m(parameter_types)</literal> is a signature of the call join
point, where <literal>R(A)</literal> is the return type of <literal>
m(parameter_types)</literal> as defined in <literal>A</literal>, or as inherited
by <literal>A</literal> if <literal>A</literal> itself does not
provide a definition of <literal>m(parameter_types)</literal>.
</para>
<para>
Continuing the example from above,we can deduce that
</para>
<programlisting><![CDATA[
R' S.m(String)
R P.m(String)
R Q.m(String)
]]></programlisting>
<para>are all additional signatures for the call join point arising
from the call <literal>t.m("hello")</literal>. Thus this call
join point has four signatures in total. Every signature has the same
id and parameter types, and a different declaring type.</para>
</sect2>
<sect2 id="method-execution-join-point-signatures" xreflabel="method-execution-join-point-signatures">
<title>Method execution join point signatures</title>
<para>Join point signatures for execution join points are defined
in a similar manner to signatures for call join points. Given the
hierarchy:
</para>
<programlisting><![CDATA[
interface Q {
R m(String s);
}
class P implements Q {
R m(String s) {...}
}
class S extends P {
R' m(String s) {...}
}
class T extends S { }
class U extends T {
R' m(String s) {...}
}
]]></programlisting>
<para>Then the execution join point signatures arising as a result
of the call to <literal>u.m("hello")</literal> are: </para>
<programlisting><![CDATA[
R' U.m(String)
R' S.m(String)
R P.m(String)
R Q.m(String)
]]></programlisting>
<para>Each signature has the same id and parameter types, and a
different declaring type. There is one signature for each type
that provides its own declaration of the method. Hence in this
example there is no signature <literal>R' T.m(String)</literal>
as <literal>T</literal> does not provide its own declaration of
the method.</para>
</sect2>
<sect2 id="field-get-and-set-join-point-signatures" xreflabel="field-get-and-set-join-point-signatures">
<title>Field get and set join point signatures</title>
<para>
For a field get join point where an access is made to a field
<literal>f</literal> of type <literal>F</literal>
on a object with declared type <literal>T</literal>, then
<literal>F T.f</literal> is a signature of the get join point.
</para>
<para>
If <literal>T</literal> does not directly declare a member
<literal>f</literal>, then for each super type <literal>S</literal>
of <literal>T</literal>, up to and including the most specific
super type of <literal>T</literal> that does declare the member
<literal>f</literal>, <literal>F S.f</literal> is a signature
of the join point. For example, given the hierarchy:
</para>
<programlisting><![CDATA[
class P {
F f;
}
class S extends P {
F f;
}
class T extends S { }
]]></programlisting>
<para>
Then the join point signatures for a field get join point of
the field <literal>f</literal> on an object with declared type
<literal>T</literal> are:
</para>
<programlisting><![CDATA[
F S.f
F T.f
]]></programlisting>
<para>The signatures for a field set join point are derived in an
identical manner.</para>
</sect2>
</sect1>
<sect1 id="join-point-modifiers">
<title>Join Point Modifiers</title>
<para>Every join point has a single set of modifiers - these include
the standard Java modifiers such as <literal>public, private,
static, abstract</literal> etc., any annotations, and the throws
clauses of methods and constructors. These modifiers are the
modifiers of the <emphasis>subject</emphasis> of the join point.</para>
<para>
The following table defines the join point subject for each kind
of join point.
</para>
<informaltable>
<tgroup cols="2">
<thead>
<row>
<entry>Join Point Kind</entry>
<entry>Subject</entry>
</row>
</thead>
<tbody>
<row>
<entry>Method call</entry>
<entry>The method picked out by Java as
the static target of the method call.</entry>
</row>
<row>
<entry>Method execution</entry>
<entry>The method that is executing.</entry>
</row>
<row>
<entry>Constructor call</entry>
<entry>The constructor being called.</entry>
</row>
<row>
<entry>Constructor execution</entry>
<entry>The constructor executing.</entry>
</row>
<row>
<entry>Field get</entry>
<entry>The field being accessed.</entry>
</row>
<row>
<entry>Field set</entry>
<entry>The field being set.</entry>
</row>
<row>
<entry>Pre-initialization</entry>
<entry>The first constructor executing in
this constructor chain.</entry>
</row>
<row>
<entry>Initialization</entry>
<entry>The first constructor executing in
this constructor chain.</entry>
</row>
<row>
<entry>Static initialization</entry>
<entry>The type being initialized.</entry>
</row>
<row>
<entry>Handler</entry>
<entry>The declared type of the
exception being handled.</entry>
</row>
<row>
<entry>Advice execution</entry>
<entry>The advice being executed.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
<para>For example, given the following types</para>
<programlisting><![CDATA[
public class X {
@Foo
protected void doIt() {...}
}
public class Y extends X {
public void doIt() {...}
}
]]></programlisting>
<para>Then the modifiers for a call to <literal>(Y y) y.doIt()</literal>
are simply <literal>{public}</literal>. The modifiers for a call to
<literal>(X x) x.doIt()</literal> are <literal>{@Foo,protected}</literal>.
</para>
</sect1>
<sect1 id="join-point-matching-summary">
<title>Summary of Join Point Matching</title>
<para>
A join point has potentially multiple signatures, but only one set of
modifiers. <emphasis>A kinded primitive pointcut matches a particular join point
if and only if</emphasis>:
</para>
<orderedlist>
<listitem>They are of the same kind</listitem>
<listitem>The signature pattern (exactly) matches at least one
signature of the join point</listitem>
<listitem>The modifiers pattern matches the modifiers of the
subject of the join point</listitem>
</orderedlist>
<para>Given the hierarchy</para>
<programlisting><![CDATA[
interface Q {
R m(String s);
}
class P implements Q {
@Foo
public R m(String s) {...}
}
class S extends P {
@Bar
public R' m(String s) {...}
}
class T extends S {}
]]></programlisting>
<para>and the program fragment:</para>
<programlisting><![CDATA[
P p = new P();
S s = new S();
T t = new T();
...
p.m("hello");
s.m("hello");
t.m("hello");
]]></programlisting>
<para>
The the pointcut <literal>call(@Foo R P.m(String))</literal> matches the
call <literal>p.m("hello")</literal> since both the signature and the
modifiers match. It does not match the call <literal>s.m("hello")</literal>
because even though the signature pattern matches one of the signatures
of the join point, the modifiers pattern does not match the modifiers of
the method m in S which is the static target of the call.
</para>
<para>The pointcut <literal>call(R' m(String))</literal> matches the
calls <literal>t.m("hello")</literal> and <literal>s.m("hello")</literal>.
It does not match the call <literal>p.m("hello")</literal> since the
signature pattern does not match any signature for the call join point
of m in P.</para>
</sect1>
</chapter>
|
