I'm working with the Android tutorial and I just got to the debugging section and I'm wondering what the purpose of a Breakpoint is. I can't tell just yet... is it actually stopping the app so I can be sure it runs up until that point, or can I set multiple breakpoints and use them as markers to "stop and go" from breakpoint to breakpoint checking my code?
A breakpoint is a place where the execution stops, and you can start inspecting the current situation in your debugger. This includes:
the point has actually been reached
the current values of all variables
the ability to change manually all variables
the current stacktrace - i.e. which methods were executed before the current one
the ability to add and execute arbitrary code
the ability to inspect the results of a method invocation, while not actually proceeding with the execution
In addition to that, you can manually step forward, line by line in your application. There are three options:
step into - enters a method which is invoked in the current line
step over - goes to the next line
step return - returns from the current method (to the method that invoked it)
You can set multiple breakpoints if you have multiple places where you want to do any of the above.
Generally speaking, a debugger is a very upgraded version of using System.out.println(..) or log.debug(..) all over the place in order to make sure certain conditions are present. (thanks to BalusC for this point)
You can definitely set multiple breakpoints. The questions answered by the breakpoint (alongside all of Eclipse's other debug tooling) include not only "did it get here" but also "how did it get here" (the stack trace) and "with what values" (you can observe the variables while the code is paused).
Related
How can I tell Eclipse to NOT jump to the first line of every file every time it jumps into a function of an object?
If I chose to step into a member function of an object, Eclipse opens the according file but instead of going straight to that function it always jumps to the first line first.
package org.diango.simcalc.pipeline; // <-- This is where the debugger "stops"
import org.apache.commons.lang.NullArgumentException;
// .. imagine more code
This is insanely annoying.
The reason is probably the same why you sometimes see "file.java:1" in stack traces as in this question: In certain situations Java generates special synthetic methods in your class automatically and these seem to have line number 1.
Eclipse has step filters for the debugger, and this can be used to filter synthetic methods.
In the preferences dialog of Eclipse, go to "Java" -> "Debug" -> "Step Filtering", and enable "Use Step Filters" and "Filter synthetic methods". Make sure to also leave "Step through filters" checked, otherwise you would effectively step over the synthetic method and the actual target method.
When debugging in IntelliJ Idea if I put a break point on a method signature it warns me about slow performance. However, what I do instead is just put it on the first executable line of code in the function and it works fine for my purposes.
My understanding is that if I see the function I will for sure see right before the first line of executable code inside the function but IntelliJ disagrees. What differences would I expect to see in with these two different debug methods?
Using method breakpoints creates a need to check every time a method is called to determine whether it needs to be breaked for every method call on the application. They also disable JVM optimizations such as method inlining.
Regardless to say this issue is not IDEA related, might even be applicable for other programming languages.
Don't have IntelliJ IDEA in front of me, but based on documentation, I think with method breakpoints you aren't actually going inside method implementation, but rather step-by-step on method calls. Which is probably the reason for performance impact, since IDEA must walk through caller stack, compared to just walking through lines of code in a single method in case of line breakpoint.
I have searched and found that indeed Eclipse does not support this 'direct' feature. But, Did I stil miss something? and Is it present in other IDEs?
Let me elaborate my question more -
if a statement falls under execution flow based on an expression evaluation, then why can't we force execute it? (without the execution of the expression).
For example consider this -
... if(bool returnsABoolean) {
<execute some statement>;
}
...
Can the execution of 'if' be skipped and the statement be executed as a 'next statement'? (I obviously can control the value of 'returnAsBoolean' in the Variables view; but can I not skip (in a controlloed manner) all the statements until a particular statement in the execution?)
Highlight the code you want to run and right-click/Execute or press Ctrl+U.
Alternatively to "Execute", use "Display" (Ctrl+Shift+D) or "Inspect" (Ctrl+Shift+I) to see the result.
Looks like you want the 'Display' view - in the Debug perspective, do :
Window -> ShowView -> Display.
You can enter Java statements to execute there (you have to select the bit of text each time that you want to execute)
http://help.eclipse.org/luna/index.jsp?topic=%2Forg.eclipse.jdt.doc.user%2Freference%2Fviews%2Fdisplay%2Fref-display_view.htm
Debugging allows you to run a program interactively while watching the source code and the variables during the execution.
So debugging is nothing but executing program but inspecting the elements during execution but you can not jump into something which is not there in execution.
You can use keyboard buttons F5,F6 ,F8 etc. (For Eclipse) and other Shortcuts during debugging for your convinience but you can't jump to something directly which is not in the execution sequence.
Debugging Shortcuts:
F5 Step into
F6 Step over
F8 Resume and will take you to the break point
Ctrl+Shift+B Toggle breakpoint
Ctrl+Shift+D Display Info of current statement
Ctrl+Shift+I Inspect the selected element
You can Skip some code by the use of breakpoint you can directly jump to specific point and avoid debugging of code which you believe works fine.Or you can jump out code snippet if you want to.
The question really was to set the Instruction pointer at will. This has been discussed and through url's i pasted on the comments above - this is not an eclipse feature (yet).
I have the following problem, we might even call it a classic one:
public void myMethod(Map<Object, Object> parameter){
someOtherObject.method(parameter);
.
.
.
someOtherThirdPartyObject.method(parameter);
}
And suddenly, in the end some method touched the input parameter Map, and I don't know where and how. Now, I know it would be desirable to make the parameter immutable, but it is not and that is the root of the problem. For instance, the methods inside myMethod are intended to perform some validations, but they do some more as well, which is wrong by design.
So, the question is how to create a breakpoint in this method where the execution pauses if an attribute of this parameter Map changes? It might be a good idea to put a conditional breakpoint after each method call, but if you have 20-odd methods, it's rather painful.
How can I debug when this input parameter is changing?
What you want appears to be called a "watchpoint". I actually didn't know this functionality existed and I used to work on the Eclipse Project!
http://help.eclipse.org/indigo/index.jsp?topic=%2Forg.eclipse.cdt.doc.user%2Ftasks%2Fcdt_t_add_watch.htm
It looks like you'll have to figure out what fields are being editted and then set a "Write" watchpoint using the help document above.
Additionally, Eclipse highlights variables which are modified, so if you step over your method calls one by one you will be able to see which one is modifying the value (and which field is being modified) because it will be highlighted (bright yellow, by default) in the "variables" tab in the "debug" perspective. Once you know which method if modifying the data you can run debug again, but this time debug the method that changes the value and just keep repeating until you find the problem.
This is a classic problem solving scenario where you start with a very large search space and systematically and methodologically narrow it down until the search space is small enough for you to locate the problem.
If you're trying to locate a spot where your map is being modified incorrectly, you might want to first start at the higher levels of the myMethod. Put breakpoints around the methods called inside the myMethod method. At each breakpoint, look at the contents of the Map. Eclipse has a variable watch panel where you can see the contents of every variable at a specific moment in time.
When you hit the breakpoint where you notice something is wrong. Stop. You now know to dig into someOtherObject.method(parameter); assuming the data was changed at it's breakpoint.
Now, someotherObject.method will likely have other methods inside it. Put your breakpoints inside this method around all of it's function calls and repeat the process. Continue repeating until there are no more methods left. Eventually, you will narrow down the problem and have the answer.
Unfortunately, there is no magic "fix my code" button for these types of problems. It just takes good, old fashioned Sherlock Holmes style investigative skills and reasoning to eliminate areas of the code that you know aren't the problem until you're left with a smaller section that allows you to get at the root cause.
If no code modification is allowed, you can
use the watchpoints method described by acattle to watch changes at this specific map instance or
have breakpoints in the Map methods modifying its state (if you want to do that for multiple instances). It does not matter that the Map code is binary only, you can still open it using Ctrl-Shift-T (Open Type), select the methods like put(...) or remove(...) in the outline view and add breakpoints using the context menu in the outline view.
Can I move the instruction pointer directly to a line of my choice (within the current method) while debugging a Java program in Eclipse (Galileo)?
It's straightforward to drag the instruction pointer to the desired line within a method in Visual Studio, but I don't see a way to do that in Eclipse (and don't find anything about it in the docs or on google).
This is possible...
http://help.eclipse.org/galileo/index.jsp?topic=/org.eclipse.jdt.doc.user/tips/jdt_tips.html
Drop to frame - When stepping through your code, you might occasionally step too far, or step over a line you meant to step into.
Rather than restarting your debug session, you can use the Drop to
Frame action to quickly go back to the beginning of a method. Select
the stack frame corresponding to the Java method you wish to restart,
and select Drop to Frame from Debug view toolbar or the stack frame's
context menu. The current instruction pointer will be reset to the
first executable statement in the method. This works for non-top stack
frames as well.
Note that Drop to frame is only available when debugging with a 1.4 or
higher VM, or the J9 VM. There are some situations where a JVM may be
unable to pop the desired frames from the stack. For example, it is
generally impossible to drop to the bottom frame of the stack or to
any frame below a native method.
This is not possible.
If you simply want to execute some code at the current place you can use the Expressions view and enter your code as an expression. The methods called by the expression evaluation will run in the current debugging context.
Moving the pointer like in Visual Studio is not possible, however workarounds are:
Going backwards to the beginning of the currently executed method:
Select the method from the debug call stack, right click -> "Drop to frame"
et voila you're back at the beginning of the method.
Now to reach your desired line select the line by clicking in it and hit ctrl+r or right click the line and select "Run to line".
These techniques are hugely helpful and reduce debugging efforts massively, enjoy!
A trick I use is to type a space in your class, somewhere safe such as in the comment line; immediately delete it and save the class. This forces the execution point to jump to the beginning of your current method. Not ideal, I admit, but it can sometimes be used as a workaround to achieve what you want.
Although in the default installation of eclipse it is not possible to do directly move the execution point like in Visual Studio, there may exist an eclipse plugin which provides that functionality somewhere. Have a search around.
I like ankon's answer best, but another option (that will only work for your specific instance -- if that) is to stop at a breakpoint on your if and modify the variable(s) evaluated in the conditional such that it returns false (from the "Variables" view, right click on a variable and click "Change Value...")
I thought that this was totally possible in older versions of eclipse, I thought I had the memory of doing it, but I guess I just implanted that memory when I worked in Visual Studio. From what I'm reading it might come to the jvm and not eclipse itself, there are pages where it's suggested that the jvm cannot handle that.
In my opinion Eclipse is many many times better than VS, I worked extensively in both and since I discovered Eclipse I was always in pain when I had to work in VS. But not having this feature is definitely hurting right now hehe.
You can jump directly to any other method call inside of the currently debugged method. Select some method call below your current instruction pointer and use "Step into selection" from the context menu.
unfortunately not possible to step forward with instruction pointer (program counter), so what you need to do instead is to introduce your own "debugging" variables that you can test on - lets say you want to step around a loop that takes too long, then add a variable and test on its increased value and then encapsulate the loop in an if with that variable. I know this is ugly, but it gets it done - or you could just develop in C++ :-)
Just right click on desired line and choose run to line.That's it...
Put the cursor on the line of your choice and either hit ctrl-R ("Run to line") or right-click and select "Run to line" from the context menu.