Java Performance Monitoring & Tuning

Getting head dump
2 ways:
1. Use jmap (which is also part of the Sun JDK 6) to dump the heap of a running application to a file using a command line like
jmap -dump:format=b,file=myheap.hprof <pid> (more…)





In generic type declaration the wildcard ‘?’ and the ‘super’ keyword are NOT allowed; following will NOT compile:
public <? super T> void go(List<T> a) {  }

However, this is perfectly valid:
public static <T extends Comparable<? super T>> void sort(List<T> list)

The type has to implement comparable of itself or its superclass. So consider java.util.Date. It implements Comparable<Date>. But what about java.sql.Date? It implements Comparable<java.util.Date> as well (java.sql.Date extends java.util.Date).
Without the super signature, SortedList would not be able accept the type of java.sql.Date, because it doesn’t implement a Comparable of itself, but rather of a super class of itself.

Extends – tells you what you can get out of a class (you get at least this, perhaps a subclass). e.g.,
public static <T> void copy(List<T> dest, List<? extends T> src)

Super – tells you what you can put into the class (at most this, perhaps a superclass).


Integration Tests

What is an integration test

Sometimes there is not a clear distinction on what is an integration test and what is a unit test.

Basic rule of thumb is that if

  1. a test uses the database
  2. a test uses the network
  3. a test uses an external system (e.g. a queue or a mail server)
  4. a test reads/writes files or performs other I/O

…then it is an integration test and not a unit test. Below is brief comparison between the two. (more…)

Resource Loading

Use either of below way given a) directory is on the classpath. b) class loading the resource is loaded by the same classloader.

1.    From ClassLoader perspective, all paths are “absolute” already – there’s no context from which they could be relative. Therefore, no need for a leading slash.
InputStream in = this.getClass().getClassLoader().getResourceAsStream(“SomeTextFile.txt”);

2.    From Class perspective, the path is relative to the package of the class unless a leading slash is included, so if we don’t want to use the current package, include a slash like this:
InputStream in = this.getClass().getResourceAsStream(“/SomeTextFile.txt”);