I'm confused with these two terms.
Also what should I do to create a file under the src folder of a Spring MVC Project?
When I create using a File object it creates the file inside C:\SpringSourceTool...
I guess this is ClassPath right?
How can I get the applicationcontext folder or root of the application whatever?
The build path is used for building your application. It contains all of your source files and all Java libraries that are required to compile the application.
The classpath is used for executing the application. This includes all java classes and libraries that are needed to run the java application. A Classpath is mandatory, the default path is . which is used if the java virtual machine can't find a user defined path. (CLASSPATH environment variable, -cp flag or Class-Path: attribute in a jar manifest)
The classpath is the conventional way to tell the (standard) Java compiler and the Java runtime where to find compiled classes. It is typically a sequence of JAR file names and directory names. The classpath used by the compiler and the runtime system don't have to be the same, but they typically should be, especially for a small project.
Buildpath is not standard Java terminology. It is the term for the richer way that a typical IDE specifies the relationship between the "projects" that make up an application. The IDE uses this to figure out the classpath and sourcepath for compiling the Java code, and the classpath for running it. The IDE also uses the build path to figure out how to package up your code and its dependencies as (for example) a WAR file.
For example, an Eclipse build path for a project includes the other projects that it depends on, and lists any additional library JARs that the project contains / relies on. It also lists the packages in the current project that downstream projects can depend on.
(If you are using Maven for your project, the IDE buildpath mechanism is secondary to the dependencies declared in the POM files. For example, using Eclipse with the m2eclipse, the buildpath is synthesized from the POM files.)
The class path is used at runtime to load compiled classes and resources.
The build path is used at compile time to find the dependencies needed to build your project.
I would like to add to Andreas_D's answer to explain that the build path is required by the IDE/compiler to locate external packages and classes used by your code. We sometimes refer to these as 'dependencies'.
NB: These external packages may be packaged inside a compressed .jar file or indeed, there may be several jar files packaged inside a 'library'. A library or group of libraries often make up a 'framework'.
If your code requires code written by others, you can import them into your class using the import command. However, this command on its own is insufficient as the compiler or IDE needs to know where those classes are located. You specify this in the build path.
The classpath on the other hand tells the JVM running your application where to find any dependencies during the actual execution of your code.
Also to note:
Classpath is for use by the JVM.
Buildpath is for use by the IDE/compiler and is a means to construct the classpath from your development environment. When you configure your buildpath via your IDE, you are also configuring a hidden file in your project called .classpath. This is used to provide the classpath to JVM at deployment.
Each Java project has its own build path that specifies all dependencies required to compile the project. Those dependencies may come from other Java projects in the workspace, from Java archive .jar files, or from folders containing .class files.
In CLASSPATH environment you need to specify only .class files (i.e., jar, zip files – Inside jar, zip files you will find only java classes) i.e. you are helping Java Virtual Machine (JVM) to find Java class files
Also what should i do to create a file
under the src folder of a Spring MVC
Project? When i create using a File
object it creates the file inside
C:\SpringSourceTool...
This is where the JVM was started, if you want to create the file else where, use relative path from here.
See this and this for more info.
Classpath (from Wikipedia):
Similar to the classic dynamic loading behavior, when executing Java
programs, the Java Virtual Machine finds and loads classes lazily (it
loads the bytecode of a class only when the class is first used). The
classpath tells Java where to look in the filesystem for files
defining these classes.
The virtual machine searches for and loads classes in this order:
bootstrap classes: the classes that are fundamental to the Java
Platform (comprising the public classes of the Java Class Library, and
the private classes that are necessary for this library to be
functional).
extension classes: packages that are in the extension
directory of the JRE or JDK,
jre/lib/ext/ user-defined packages and
libraries
By default only the packages of the JDK standard API and
extension packages are accessible without needing to set where to find
them. The path for all user-defined packages and libraries must be set
in the command-line (or in the Manifest associated with the Jar file
containing the classes).
Simply put - while your program is running, the JVM loads classes only as needed. When a class is needed, the JVM will depend on the classpath to it know where to load the bytecode from (i.e.: .class files).
Build path, on the other hand, is typically used by an IDE, such as Eclipse, to know where to look for additional libraries that are required to compile a project's source code. Build path isn't used during runtime.
Related
When setting up a Java web application using Tomcat as an application server I often get confused about when libraries are available. Through some discussion on Stack Overflow, I have learned that some libraries (.jar) files are available at runtime, while others are available at compile time. Often I will get errors and will resolve them by trial and error, placing jar files in different directories until the application runs or compiles. It was recently pointed out to me that you can make .jar libraries available at runtime via the WEB-INF/lib folder. I started thinking about this and had a few question. I have read up on this topic in the past and haven't found a source that puts the information into a context I easily understand and retain.
Is there a compile time classpath and a runtime classpath you can set for a project?
a. Is classpath even an applicable term for discussing libraries available at runtime?
Is WEB-INF/lib the only way to make libraries available at runtime? What about the lib folder in Tomcat is this available at runtime?
How does this relate to classloaders? I know that a hierarchy of classloaders is created. Are these strictly for Runtime operations?
The compile classpath is the classpath used to compile your Java source files (using javac -cp ..., or your IDE). Every class referenced in the source file must be present in the compile classpath, else the compiler will complain that it can't find the class.
Once you have compiled the classes, you can run a program using them (using java -cp ...). Obviously, the libraries on which your source code depends directly should be in the runtime classpath. But that's not all. If you depend directly on CoolLibrary.jar, and this library internally depends on Guava.jar, then Guava.jar must also be in the runtime classpath, although it was not needed when compiling.
Webapps are a bit special. The servlet specification specifies that the classpath used to execute the webapp is composed by the WEB-INF/classes directory of the deployed webapp, and of all the jars contained in WEB-INF/lib. All the webapps also have access to the native servlet and JSP jars, which are directly provided by Tomcat. In reality, Tomcat's internal classes (like the implementation classes of the servlet-api interfaces) are also available to the webapp, but relying on these classes is not a good idea, since it would tie your webapp to tomcat.
Talking of the runtime classpath, in the case of a webapp, is a bit of a simplification. In reality, every webapp's classes are loaded dynamically by a specific classloader by tomcat. And this webapp classloader is a child of the tomcat's classloader. So, in theory, you could place the webapp jars in Tomcat's classpath directly, but this would mean that all the webapps would share these libraries, and that you would have problems undeploying and redeploying webapps. The goal of having a specific classloader per webapp is to be able to have, in the same JVM, an app relying on Guava 11.0, and another one relying on Guava 12.0, for example.
For more information about tomcat classloaders, read the documentation.
in eclipse, you have the java build path which include the libraries during compile time, and you have order and export, which is for the runtime.
only the tomcat libraries available by default
I am beginning to use java packages like HTMLParser, I have downloaded it and finding that there are many files and directories in it.
I wander, where to place them in my linux system? Is there a convention or a standard?
The quick and dirty answer is "anywhere on the classpath", where the classpath is set either as a system property on the client machine (not recommended), as a temporary system property for the CLI session used to start the JVM (workable from a startup script), or as a commandline parameter to the JVM (usually the preferred choice).
First and second set the CLASSPATH environment variable, see the JDK or JRE documentation for the exact syntax and your operating system's and/or shell scripting documentation as well. Third uses the -cp commandline variable to the Java runtime and compiler, see their documentation for exact syntax.
Where to place the files on the filesystem? For development purposes I typically use a central folder on my computer containing all such libraries and link to that from my IDE or other development environment. For deployment/packaging to end users, it is traditional to have a "lib" subfolder to the product folder that contains all distributable content, and put the jar files in that.
Java packages come in two forms. Source code - all the files and directories you mention - and packaged as jars. A common convention in Java projects is that the project has a lib directory that contains all the jars that the project depends on. These projects often use a shell script which adds all the jars to the Java classpath prior to executing the project code.
However many projects are switching from this method of dealing with dependencies to using a build tool like Apache Maven which automatically handles dependency management. Other alternatives include Ivy or Gradle. For an introduction see the 5 minute introduction to Maven or the Maven 3 tutorial.
Here you write a pom.xml (project object model file) which specifies which libraries (jars) your project uses. Maven then stores all the jars for your different projects in a .m2 directory in your local directory, keeping track of where it obtained them, and their versioning information.
This makes developing much easier as you do not need to create the lib directory or manually manage dependencies. You also avoid a lot of the complexities of setting the classpath, as Maven automatically does this for you during common lifecyle stages such as compilation and test. Recent versions of Eclipse can read the Maven pom and automatically configure your classpath from it.
Once you have built the project, Maven can also help create "fat jars" that contain all the jars your project depends on, via the assembly plugin or the Shade plugin. This makes distributing the code easier when you are building an executable that you want someone to use. If you are distributing a jar, then your pom.xml describes the dependencies of your project, avoiding the need to distribute the jars it depends on.
For laying out files in general on a Linux system consult the Linux Filesystem hierarchy standard.
What are the different ways that java programs gain access to external libraries. There is setting a classpath, modifying the build or build path, but I've seen other ways of adding jars.
Why do some libraries have to be added to the classpath while others do not. For example I'm using JSF, WTP tools, and other extra libraries but they are not in my buildpath when I view the build path of my project.
The classpath is used to find classes when executing a Java program. The build path is used when Eclipse is compiling a Java program.
The Java Build Path is just an Eclipse thing. It's where Eclipse finds the classes needed to compile and run the classes of the project. It's thus both th compile and the run classpath.
In the case of a webapp, the webapp runs inside a Java EE web container. The web container gives access to standard Java EE classes (javax.servlet, etc.). Moreover, all the jars in WEB-INF/classes are automatically included in the classpath of the web app. So Eclipse doesn't need you to specify them in the Java Build Path. They're included automatically.
On development time.
A build path is one where you can explicitly point to third party software / jars.
By default not all third party software are added into your classpath, hence you may have to explicitly add that to your path.
On runtime.
On the other hand when you run your applications from the command line, you would prefix the classpath by using -cp to specify the third party jars.
For example in web projects you would add it to your web-inf library when you deploy.
A classpath is simply an array of classpath entries (IClasspathEntry) that describe the types that are available. The classpath is an environment variable that tells where to look for class files and it is generally set to a directory or a JAR (java archive) file.
The Java build path is reflected in the structure of a Java project element. You can query a project for its package fragment roots (IPackageFragmentRoot). The build path is the classpath that is used for building a Java project (IJavaProject).
I've downloaded a new api for Java that accesses excel files, but am unsure on how to install it so that it can be imported for use in my program. Help is appreciated. Thanks
To the point: just put it in the classpath.
A classpath is basically a collection of disk file system paths to a root folder where all classes are located like /path/to/package/root and/or paths to the JAR file itself like /path/to/file.jar. You can specify multiple paths in the classpath by a separator character. In Unix based systems like OS X the separator character is the colon : (on Windows it's the semicolon ;).
How and where to specify the classpath depends on how you're compiling and executing the program.
If you're using plain javac to compile the program, then use the -cp argument to specify the compile time classpath. Or if you're using an IDE, then add it to the project's Build Path (which covers both the compile time and runtime classpath).
If you're using java to execute the program as a simple .class file, then use the -cp argument the same way. If you're using java -jar (or doubleclicking the file in some platform specific UI explorer) to execute the program as an executabele .jar file, then you need to specify it in Class-Path entry of the JAR's MANIFEST.MF file. This one can be relative to the JAR file's location.
You don't really have to "install" it - you just have to put it inside the Classpath. For example, if you're using Eclipse, you can right-click on your project, select something like "build path"->"configure build path", then libraries.
That depends on the tools you are using for development. Basically it will have to be included on your classpath for your IDE project for development, and in your runtime classpath at deployment time.
How to accomplish this in development is specific to your project configuration, IDE and how you store dependent jar files in your development environment (i.e. shared lib directory, maven, project lib folder ...).
I am using Netbeans IDE for a java project. In this project i need a jar file "htmlunit-2.6.jar".
I have included this jar file in the project libraries folder. I have instantiated one of its class "WebClient" but this class needs other classes of "commons-httpclient-3.1.jar" file.
Now I have also included "commons-httpclient-3.1.jar" file in the project libraries folder. But when I compiled my source file, it throws
ClassNotFoundException: org.apache.commons.httpclient.auth.CredentialsProvider
Kindly tell me how to handle this situation when one class in one jar file needs other classes in other jar file.
Simply put the required jar files on the classpath at compile-time and it should work. If you were doing it from the command-line then it would look like this:
javac -cp jar1:jar2 my.Application
If you are using NetBeans then you need to tell NetBeans that both of the JARs are on your classpath. It will be definable in a Project > Properties wizard as described here and also here from the tutorial
The ClassNotFoundException tells you that your libraries have some dependencies that you don't have included in your classpath at runtime. Your source is OK, because if you have used something not available, NB will tell you this at compile time (or before when editing).
So, welcome in the "dependency hell" of Java. For small projects you will be able to check all dependencies by hand with readme files, docs, etc and put them in the project config as oxbow_lakes said. For bigger things look at maven. It will do (most) everything for you !
(Maven is available in NB6)