Filtered extraction from zip file using ZipInputStream and Java 8

By dave on May 16, 2015

In the example below we build on the Reading a zip file from java using ZipInputStream page to provide basic filtering. This filtering is provided by the filteredExpandZipFile method taking a Predicate. Every ZipEntry is passed to the predicate, but only ones that match (predicate returns true) are included. Note that the size of an entry cannot be accurately determined in all cases, so it is not safe to perform validation on the this field.

Using Future and ConcurrentMap for a lazy cache

By dave on May 10, 2015

In this example I demonstrate how to use ConcurrentMap along with Future to generate a lazy cache. Concurrent maps are a good choice in some situations, where absolute atomicity can be traded off for performance, some level of control over ordering of events has been traded for performance. Let’s assume for the sake of example that we have an object that is heavy to create. Further, it has to be created on a single thread because it calls into a single threaded maths library.

Writing a zip file in java using ZipOutputStream.

By dave on April 27, 2015

Following on from one of our popular articles on Reading a zip file from java using ZipInputStream we’ve put together a new article on how to create a zip archive using Java. The below example uses ZipOutputStream to create a zip file from all the items in a directory. Zip files are written slightly differently to a normal stream in that each entry is put into the stream one at a time.

Old Generation GC and cache performance?

By dave on April 25, 2015

This is a question as much as a discussion, I can’t find a lot of detail on the state of play in this area and would really welcome any feedback or corrections. Please don’t read this as a negative article as that is not how it is intended; it’s in the optimisation section but I’m not sure what impact it really has. One thing for sure, for 99% of systems you probably don’t have to worry about what’s going on here at all.

Formatting dates in Java with DateFormat

By dave on March 22, 2015

There are several ways to format dates in Java, but by far the easiest is to use DateFormat. Creating a DateFormat is very similar to NumberFormat that we saw on the previous page. Here are the static factory methods called directly on the DateFormat class: getDateInstance(..) getDateTimeInstance(..) getTimeInstance(..) There are several overloaded versions of each method above. We will not cover Locale here as that will be covered in another article on java timezone support.

Setting up a database realm for role based security

By dave on March 22, 2015

Following on from Setting up role based security in tomcat, we now switch from using a memory realm to one backed by a database. Memory realms are great for testing but in any real application is would probably not be acceptable. Normally user credentials are stored in a database, so for this purpose there is a realm based on a datasource. Depending on your view of things, you will either edit server.

Simple uses of CountDownLatch

By dave on March 22, 2015

CountDownLatch provides a means of waiting for a number of asynchronous events before proceeding. In order to do this one constructs a latch providing the event count. Then one thread would normally call await Whilst the other thread calls countDown. Once the count reaches zero the await call returns and the latch is set. If the call to await happens after the latch is set it returns immediately. In our example we need to wait for a thread to initialise before proceeding.

The background, string formatting in java

By dave on March 22, 2015

Over the years there have been no shortage of ways to format a string in java. What with the + operator, StringBuffer, StringBuilder, String.format(..) and various specialised formatters for numbers and dates we sometimes feel a little spoilt for choice. But how do they all work and what are their advantanges / disadvantages?  StringBuffer - a hang up from times gone by! StringBuffer is a synchronized object! Yes, everything you do with it will cause synchronization.

Using tomcat role based security in JSP"s or servlets

By dave on March 22, 2015

Following on from Setting up role based security in tomcat we now look at accessing the realm security information from code. Although tomcat takes care of authenticating users at the right time, there are still times when we need to programatically access the credential information. For example the following snippet from userProfile.jsp is a mixed mode page In that anyone can view the page, but some users with manager role see more information.

Using Java"s XMLStreamReader PULL XML Parser

By dave on February 17, 2014

In this entry I show how to use the inbuilt Java XMLStreamReader PULL parser class to read an XML file. The XML stream libraries are PULL based XML parsers that do not load the whole document into a memory structure, so therefore are more suited to large volumes of XML. Below is an example XML file for a zoo, it contains Animal data types that have both attributes and data. It is kept simple for the sake of example.

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