JavaEE Training Note - Day 01

The training content of the first day of this one-month training is some genaral concepts of JavaEE development. It also covers some basic introduction to some broadly used library/framework including Spring Framework, Hibernate ORM, etc. Besides, some trivial of Java programming language are also recapped in today’s content.

Web Development in JavaEE

Three levels of a web site

To divide large web programs to several part by functionality and to arrange the classes, a common approach is the following one.

Persistence Level

The level that store and retrieve data between memory and database (or other persistence). Classes in this level are generally called DAO, the Database Access Object.

Business level

The level in which the application performs specific actions to persisted data as a demand of user (requesting from upper level). Classes in this level are named as Service. Generally, this level interacts with multiple DAOs.

Presentation level

The level which the end user can view and interact with directly, which is UI (User Interface). It can be either a web page (including the JSP page for rendering or other template methods), a mobile application communicate with server via web API, a WeChat official accounts or mini-program, etc.

Between these levels, a DTO, Data Transfer Object is generally used to transfer data.

Some Trials in Java

Java is a mature object oriented programming language with about two-decade involvement. A great set of features were added after its initial release during last time.

initialization block

Initialization block is used to initialize static fields of class or fields of object and can be categories as two types: initialization block and static initialization block;

Initialization block is used for initial non-static fields of object. It is executed before the first line of constructor (excluding the passible super() expression). It will be executed on each time when a new object of this class is created. When multiple initialization blocks are presented within a single class, they will be executed from top to bottom sequentially. For example:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

class InitBlock {
    private String field1;
    private Date today;
    {
        // first block
        field1 = "Field 1";
    }
    {
        // second block
        today = new Date();
        System.out.println(this.today);
    }
    public InitBlock(){
        // constructor
    }
}

Static initialization block is used to initialize the static fields. It will be executed on the first time when the class is been loaded into JVM. For example:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

class InitBlock {
    private static String field1;
    private static Date today;
    static {
        // first block
        field1 = "Field 1";
    }
    static {
        // second block
        today = new Date();
        System.out.println(InitBlock.today);
    }
    public InitBlock(){
        // constructor
    }
}

summary of usage of this

As following list:

• this is a reference in Java which represent the current object itself;
• this is invocable and refers to constructors in this scenario, like this();;
• this can be used to clarify or specify a variable when which is a field of an object. For example: this.field1;
• thiscan be used to specify the current object in nested class by prefixing of the class name, like Class1.this;

annotation

annotation is a new feature added in JDK 1.6, which provides a way for programmer to add extra meta data to a class, a method, a field, a variable, a parameter or a package. The annotated data can be retrieved at both compile time and execution time. The reflection API in Java provided approach to use these meta data at execution time.

The most common one defined in JDK is @Override, which emphasizes that the current method is intended to override an corresponding method. It is widely used in Spring Framework and Hibernate in recent release targeting at JDK 1.6 or later.

Libraries & Frameworks

Spring Framework

Spring Framework is a container to manage Java object. Some keywords related to Spring Framework ist as follows:

IoC, Inversion Of Control. This is a general concept describing the phenomena that a object method which use the function provided by another class has no control logic related to the lifecycle of the function provider object.

DI, Dependency Injection, an approach to achieve the IoC function.

AOP, Aspect Oriented Programming. Cut the core business process into multiple layers and insert different process logic into different layers as steps applied to each transaction. For example, when a web server process an HTTP request and response to it, though the process may vary from each specific request, the following steps are applicable to all of them, including the request parsing step, the request logging step, the security check step, the parameter abstraction step, the business logic processing step, the content generating step, etc. Each step can be treated as an aspect of the process and connected by the framework.

Besides the core features, Spring also provides some other components to provides more functionalities. The Spring MVC provides Model-View-Controller pattern management like the Structs does. The Spring Security provides authentication features like the Sherio does.

Hibernate ORM

ORM is an abbreviation of object-relational mapping. It is used to resolve the problem of the mismatch of in-memory object of object-oriented language like Java and persisted data record of relational database like MySQL.