This page describes the standards used for Apache ServiceMix code (java, xml, whatever). Code is read by a human being more often than it is written by a human being, make the code a pleasure to read. If you're using Eclipse, configuration files for your IDE can be found at http://svn.apache.org/repos/asf/servicemix/smx3/trunk/etc/eclipse/

Indentation

Java

Lets follow Sun's coding standard rules which are pretty common in Java.

http://java.sun.com/docs/codeconv/

http://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.html

• 4 characters indentation
• No tabs please!

Correct brace style:

public class Foo {
    public void foo(boolean a, int x, int y, int z) {
        do {
            try {
                if (x > 0) {
                    int someVariable = a ?  x : y;
                } else if (x < 0) {
                    int someVariable = (y + z);
                    someVariable = x = x + y;
                } else {
                    for (int i = 0; i < 5; i++) {
                        doSomething(i);
                    }
                }

                switch (a) {
                    case 0:
                        doCase0();
                        break;
                    default:
                        doDefault();
                }
            } catch (Exception e) {
                processException(e.getMessage(), x + y, z, a);
            } finally {
                processFinally();
            }
        } while (true);

        if (2 < 3) {
            return;
        }

        if (3 < 4) {
            return;
        }

        do {
            x++
        } while (x < 10000);

        while (x < 50000) {
            x++;
        }

        for (int i = 0; i < 5; i++) {
            System.out.println(i);
        }
    }

    private class InnerClass implements I1, I2 {
        public void bar() throws E1, E2 {
        }
    }
}

XML

• Use 4 characters. This is to allow IDEs such as Eclipse to use a unified formatting convention
• No tabs please!

Interfaces

All methods of an interface are public abstract, therefore it is not necessary to specify public abstract modifiers. Similarly all fields are public static final.

However this behavior works best with most tools and IDEs and seems to be common practice so we see no reason to disallow this practice. e.g:

public interface MyInterface {
    public static final int MY_INTEGER = 0;

    public abstract void doSomething();
}

This has the added advantage that the interface can be converted into an abstract class (and copy and paste individual definitions) without changing anything.

Preferably add public/static/final to constants, and public/abstract to methods, but it's not mandatory. However, if it's there, don't take it out.

Exceptions

• A situation is only exceptional, if the program can not handle it with reasonably effort. Wrong input data should be an expected situation of the regular code, that could be handled gracefully.
• The intention of exception-handling is to separate real error-handling from the regular part of the code, so don't force the caller to mix it with unnecessary exceptions.
• Only if your code really has a problem to continue e.g., when a parameter is invalid, feel free to throw an exception!
• Do NOT throw an exception, if you only suppose the caller of your code could have a problem with a special result. Try to return a special result value instead e.g., null, and let the caller decide with a regular if-else-statement. If the caller really has a problem, HE WILL throw an exception on his own.
• But if your code throws an exception, even though it has no real problem and it could continue without an exception and return a special result value, you forestall the decision of the caller, whether the special result is really an error or not.
• If you throw an exception, where the caller would decide that it is no error in the context of the caller, you force the caller to write an exception handler in his regular part or to abort i.e., you force the caller to mix regular code with exception handling. That is the opposite of the intentention of exception handling.
• Bad example:
  java.lang.Class.forName(String) throws ClassNotFoundException
  In most programs/situations it is an error if this method does not find the class, therefore it throws an exception and forestalls the decision of the caller.
  But maybe there is a program that should check a list of class names, whether the classes are present or not. Such a program is forced to mix its regular code with error handling of an exception, that is no error at all in that context.
  The method should return a special result value instead: null. Many callers of that method have expected that situation and therefore are not in an unexpected situation/exceptional state. They could decide the situation on their own.
• Only throw checked exceptions (not derived from RuntimeException), if the caller has a chance to handle it.
• Exceptions that signal programming errors or system failures usually cannot be handled/repaired at runtime -> unchecked exception.
• If your code really has a problem to continue e.g., when a parameter is invalid, throw an unchecked exception (derived from RuntimeException) and do NOT throw a checked exception, because if not even your code can handle the problem, in the very most cases the caller has no chance to handle the problem, too. Instead there maybe somebody somewhere in the highest layers who catches all RuntimeException's, logs them and continues the regular service.
• Only if it is not possible to return special result values cleanly, use checked exceptions to force the caller to decide the situation. The caller should deescalate the situation by catching and handling one or more checked exceptions, e.g. with special result values(?) or by escalating with an unchecked exception, because the situation is an error, that can not be handled.
• Checked exceptions are an official part of the interface, therefore do not propagate checked exceptions from one abstraction layer to another, because usually this would break the lower abstraction. E.g. do not propagate SQLException to another layer, because SQLExceptions are an implementation detail, that may change in the future and such changes should not affect the interfaces and their callers.
• Never throw NullPointerException or RuntimeException. Use either IllegalArgumentException, or NullArgumentException (which is a subclass of IllegalArgumentException anyway). If there isn't a suitable subclass available for representing an exception, create your own.

Package Naming

• Package names are lowercase.
• Package names should only contain alpha-numberic characters.
• Package names should be suffixed with the name of the module in which they are defined

For example, if the module name is common, then the base package name should be org.apache.geronimo.common.

Note: This is more of a guideline than a rule, as some modules simply can not follow this convention, but where applicable they should.

Imports

• Should be fully qualified e.g. import java.util.Vector and not java.util.*
• Should be sorted alphabetically, with java, then javax packages listed first, and then other packages sorted by package name.

IDE Auto-Formatting

• Eclipse users can
• use Source -> Organise Imports to organize imports
• use Source -> Format to format code (please use default Eclipse formatting conventions, which are as above)
• IntelliJ users can
• use Tools -> Organise Imports to organize imports
• use Tools -> Reformat code to format code (uses the code style setting in IDE options)

The eclipse formater settings are available here.

JavaDoc Tags

@version Should be: @version $Revision$ $Date$

@author Should not be used in source code at all.

Unit Test Cases

• Use the naming scheme *Test.java for unit tests.
• Do not define public static Test suite() or constructor methods, the build system will automatically do the right thing without them.

Logging

• Log as much as necessary for someone to figure out what broke
• Use org.apache.commons.logging.Log rather than raw Log4j
• Do not log throwables that you throw - leave it to the caller
• Use flags to avoid string concatenation for debug and trace
• Cache flags (especially for trace) to avoid excessive isTraceEnabled() calls

Levels

• Use trace level for detailed/diagnostic logging
• Use debug level for things an application developer would need to know
• Use info level for things an administrator would need to know
• Use warn level for things indicating an application or transient problem
• Use error level for things indicating a problem with the server itself
• Use fatal level for things that mean this instance is compromised

Example

private static final Log log = LogFactory.getLog(MyClass.class);

public void doSomeStuff(Stuff stuff) throws StuffException {
    boolean logTrace = log.isTraceEnabled();
    try {
        if (logTrace) {
            log.trace("About to do stuff " + stuff);
        }
        stuff.doSomething();
        if (logTrace) {
            log.trace("Did some stuff ");
        }
    } catch (BadException e) {
        // don't log - leave it to caller
        throw new StuffException("Something bad happened", e);
    } catch (IgnorableException e) {
        // didn't cache this as we don't expect to come here a lot
        if (log.isDebugEnabled()) {
            log.debug("Ignoring problem doing stuff "+stuff, e);
        }
    }
}