Java – How to set tomcat work at debug mode

javatomcattomcat5.5

I deploy a application to tomcat with context xml. I want the tomcat work at debug mode, I means if I change something inside a function, like change
String a="123";
to
String a="456";
tomcat should get the change without reload the application.

The web01.xml under %CATALINA_HOME%/conf/Catalina/localhost

<?xml version='1.0' encoding='utf-8'?>
<Context docBase="d:/document/workspace3.6/web01/WebContent" path="/web01" reloadable="false" debug="1" ></Context>

but now tomcat didn't worked as I expected, if I replace the old class file with new version, I must restart tomcat to get the change.

Why tomcat didn't reload the class,How should I do to let it work as debug mode?

I am not use Eclipse now. and I don't want to set reloadable="true", this will reload the entire application if class changed.

I used tomcat5.5.23

Best Answer

You're actually confusing the notions of "debugging" and hot deploy. You can configure Tomcat for debug mode, and then remotely debug your application running inside tomcat such that when you add a break point in your code, the debugger will jump to that breakpoint and halt execution.

What you actually need there is having the possibility to hotdeploy an application. With tomcat, if you modify the .java files and then copy them to the working directory of tomcat, you'll get exactly what you want, namely the ability to change something in a class and have the running tomcat-deployed application take it into account without redeploying the whole application. You can automatize this by configuring your tomcat application context (either in the tomcat server.xml file or in a project specific context.xml file) for your application to have as working directory the directory where your project code gets compiled.

here's an actual example:

Let's say you have a maven project in the directory c:\myProject. You'd have source files in the c:\myProject\src, and then when compiling it you'd get the war file and an exploded directory of the war file content in the c:\myProject\target\myProject.war and respectively c:\myProject\target\myProject. Now, if you configure your tomcat such that for the myProject tomcat context, youd have the working directory configured as c:\myProject\target\myProject, then each time you modify a .java file, the .class corresponding file will be updated in the target (and now also working) dir, and tomcat will take it into account.

I've actually used such a setup to develop with tomcat, but it's not the best. First off tomcat will hotdeploy only certain modifications, such as when you modify something in the body of an existing method. Other modifications will not be taken into account, such as adding a new method - for this you have to do a full redeploy to have it taken into account.

A far better solution is to use maven with the maven jetty plugin. This thing really works as you want: any modification you do to a class of jsp file will me immediately taken into account, and visible in the running app inside jetty.

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Update:

Some 10 years later perhaps the best way to test a private method, or any inaccessible member, is via @Jailbreak from the Manifold framework.
@Jailbreak Foo foo = new Foo();
// Direct, *type-safe* access to *all* foo's members
foo.privateMethod(x, y, z);
foo.privateField = value;
This way your code remains type-safe and readable. No design compromises, no overexposing methods and fields for the sake of tests.

If you have somewhat of a legacy Java application, and you're not allowed to change the visibility of your methods, the best way to test private methods is to use reflection.

Internally we're using helpers to get/set private and private static variables as well as invoke private and private static methods. The following patterns will let you do pretty much anything related to the private methods and fields. Of course, you can't change private static final variables through reflection.

Method method = TargetClass.getDeclaredMethod(methodName, argClasses);
method.setAccessible(true);
return method.invoke(targetObject, argObjects);

And for fields:

Field field = TargetClass.getDeclaredField(fieldName);
field.setAccessible(true);
field.set(object, value);

Notes:
1. TargetClass.getDeclaredMethod(methodName, argClasses) lets you look into private methods. The same thing applies for getDeclaredField.
2. The setAccessible(true) is required to play around with privates.

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Map<String, String> map = ...
for (Map.Entry<String, String> entry : map.entrySet()) {
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}

On Java 10+:

for (var entry : map.entrySet()) {
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