Understand Persistence Context Collision in JEE

When deal with persistence layer in JEE application, persistence context is the magic that make your entity instances different from a normal POJO. Persistence context manages the entity instances to make it finally synchronized with database. The persistence context collision happens when invoking...

How to setup a maven JEE project in Eclipse

Let's suppose you are new to JEE programming. After reading oracle official JEE tutorial document for a few days, you decide to get your hands wet.  You  download and install a JEE container, GlassFish 4 in this tutorial,  to your PC. You have JDK,maven and Eclipse all ready. So, what's...

Low-level synchronization in Java

Low level synchronization is the mechanism used before package java.util.concurrent was introduced into Java 1.5. You may not use it since the modern Java provide more high level sync mechanism since Java 1.5, but understand low-level sync can help you understand high-level counterparts like java.util.concurrent.locks.Condition,...

Understand Glassfish JDBC configuration by inspecting JDBC DataSource API

When configure DataSource on JEE container Glassfish, There are 2 things need to be set, JDBC Resource and JDBC Connection Pools. What's the difference and why are these two items? What's the relation between them? We can answer that by inspecting the Java JDBC API, mainly in package javax.sql 1. How...

Break down class "Files" of java 7 NIO.2

Since Java 7, File related IO operation has been redesigned. The following 3 classes/interfaces are key for file related operations: Path,Paths, and Files. They are all in package java.nio.file. Among all these 3, Files is the most fundamental one. 0. Brief about Path and Paths Paths is a concrete classes...

How to understand < ? super Child> in Java Generics

In Java, polymorphism does NOT apply to generic types. So suppose class Child extends from class Parent, then the following 2 lines, only the first can pass compilation. Parent var = new Child(); // compiler is happy with this ArrayList<Parent> myList = new ArrayList<Child>(); // compilation errorThe way to bring the idea of inheritance into Java generic types is using <? extends...

How to understand < ? extends Parent> in Java Generics

In Java, polymorphism does NOT apply to generic types. So suppose class Child extends from class Parent, then the following 2 lines, only the first can pass compilation. Parent var = new Child(); // compiler is happy with this ArrayList<Parent> myList = new ArrayList<Child>(); // compilation errorThe way to bring the idea of inheritance into Java generic types is using <? extends...

Glob in Java file related match

Globs are not regular expression. Glob is simpler and earlier than regular expression. For historical reasons, glob are more used as file name or file path filtering. In Java 7 NIO package has 2 places that glob appears as file names or file path filter. One is when you create a PathMatcher to test if a java.nio.file.Path instance matches a patten likePath path = Paths.get("abc.java");PathMatcher...

A brief in Java string format

When to format a Java String using System.out.printf or System.out.format, The syntax for the format is : %[arg_index$][flags][width][.precision]<conversion_char>Here are more explanation:arg_index    An integer with suffix '$'. begin with 1, not 0. The position of the args. flags - Left-align, default is right-aligned + Include +/-before number 0 Pad with zeros before number...