1 Introduction
In order to debug an application, it is sometimes useful to know where a given object comes from, or where was it sent to. An object tracer describes the origin of an object and also the places where the object was provided as an argument or returned as a result.
2 Goals
Implementation of a tracer of Java objects. The tracer can be invoked from any point of a Java program, accepting an object as argument. The tracer should immediatly log on System.err the history of that object in terms of the place where it was created, and all places where it was provided as an argument or returned as a result.
The tracer is called using the following form:
ist.meic.pa.Trace.print(object)
This means that y o u need to implement the class ist.meic.pa.Trace withastatic method that accepts one object as argument.
As a result of calling the previous method, the tracer must then present the point of instantiation and the points of call/return in the history of the object, namely:
- The constructor call used to create the object, including le name and line number.
- All method calls where the object was used as an argument, including le name and line number.
- All method calls where the object was returned, including le name and line number.
It should be possible to invoke the tracer in several di erent points of the (traced) program, with several di erent objects. The trace information for that object should follow the time sequence of operations on that object, from the oldest one to the most recent one.
3 Interface
The tracer presents the object history based on a textual representation that is printed on the console. As an example, consider the following Java class de ned in le Test0.java:
1 import ist.meic.pa.Trace;
2
3 class Test {
4
5 public Object foo() {
6 return new String("Foo");
7 }
8
9 public Object bar() {
10 return new String("Bar");
11 }
12
13 public Object identity(Object o) {
14 return o;
15 }
16
17 public void test() {
18 Trace.print(foo());
19 Object b = bar();
20 Trace.print(identity(b));
21 }
22 }
23
24 public class Test0 {
25
26 public static void main(String args[]) {
27 (new Test()).test();
28 }
29 }
The execution of this Java class le produces the following output:
$ java Test0
Tracing for Foo is nonexistent!
Tracing for Bar is nonexistent!
However, by using Javassist, it is possible to transform the bytecode of the class le in a way that allows much more information to be printed. The bytecode transformation is done at load time, by the class TraceVM.
Using this class, the example becomes:
$ java ist.meic.pa.TraceVM Test0
Tracing for Foo
<- java.lang.String(java.lang.String) on Test0.java:6
<- Test.foo() on Test0.java:18
-> ist.meic.pa.Trace.print(java.lang.Object) on Test0.java:18
Tracing for Bar
<- java.lang.String(java.lang.String) on Test0.java:10
<- Test.bar() on Test0.java:19
-> Test.identity(java.lang.Object) on Test0.java:20
<- Test.identity(java.lang.Object) on Test0.java:20
-> ist.meic.pa.Trace.print(java.lang.Object) on Test0.java:20
Note, in the previous example, that the arrows show the direction of the control flow. The arrow <- indicates that the object was returned, either from calling a constructor, or from calling a method. The arrow -> indicates that the object was provided as argument to a constructor or to a method call. After printing the arrow, the tracer prints the invoked constructor or method, and the le name and line number where the invocation was done.
Asasecond example, consider the following le Test1.java:
1 import ist.meic.pa.Trace;
2
3 class Test {
4
5 public Object foo() {
6 return new String("Foo");
7 }
8
9 public Object bar() {
10 return foo();
11 }
12
13 public Object baz() {
14 return bar();
15 }
16
17 public void test() {
18 Trace.print(foo());
19 Trace.print(bar());
20 Trace.print(baz());
21 }
22 }
23
24 public class Test1 {
25
26 public static void main(String args[]) {
27 (new Test()).test();
28 }
29 }
Running the previous class le using the bytecode transformation implemented by TraceVM, produces the following output:
$ java ist.meic.pa.TraceVM Test1
Tracing for Foo
<- java.lang.String(java.lang.String) on Test1.java:6
<- Test.foo() on Test1.java:18
-> ist.meic.pa.Trace.print(java.lang.Object) on Test1.java:18
Tracing for Foo
<- java.lang.String(java.lang.String) on Test1.java:6
<- Test.foo() on Test1.java:10
<- Test.bar() on Test1.java:19
-> ist.meic.pa.Trace.print(java.lang.Object) on Test1.java:19
Tracing for Foo
<- java.lang.String(java.lang.String) on Test1.java:6
<- Test.foo() on Test1.java:10
<- Test.bar() on Test1.java:14
<- Test.baz() on Test1.java:20
-> ist.meic.pa.Trace.print(java.lang.Object) on Test1.java:20
Assignment is to implement the classes ist.meic.pa.Trace and ist.meic.pa.TraceVM. The rst one is used to print the history of an object. The second one is used to transform the bytecode of all classes loaded as a result of loading its rst argument.
In order to implement the required output format, y o u should consider the following templates:
When there is no tracing information:
Tracing for object is nonexistent!
When there is tracing information:
Tracing for object
followed, in the case of a constructor or method call returning object as result, by
<- behavior on file :line
or, in the case of a constructor or method call using object as argument
-> behavior on file :line
In the previous templates, object is the printed representation of an object, as produced by its toStringmethod, while behavior is the description of a constructor or method, as implemented b y Javassist API method getLongName. Finally, file is the name of the le and line is a number.
4 Code
Your implementation must work in Java 6 and should use the bytecode manipulation tool Javassist, version 3.18.1-GA.
The written code should have the best possible style, should allow easy reading and should not require excessive comments. It is always preferable to have clearer code with few comments than obscure code with lots of comments.
The code should be modular, divided in functionalities with speci c and reduced responsibilities. Each module should haveashort comment describing its purpose.
You must implement a Java class named ist.meic.pa.Trace that must provide, at least, the method with signature:
static public void print(Object object)
The previous method prints on System.err the trace of the object provided as argument, following the format described previously.
You must also implement a Java class named ist.meic.pa.TraceVM, containingastatic method main that accepts, as arguments, the name of another Java program (i.e., a Java class that also containsastatic method main)) and the arguments that should be provided to that program. The class should (1) operate the necessary transformations to the loaded Java classes so that objects are traced during the execution of the program, and (2) should transfer the control to the main method of the program.