The original and reference implementation Java compilers, virtual machines, and class libraries were developed by Sun from 1995. As of May 2007, in compliance with the specifications of the Java Community Process, Sun relicensed most of its Java technologies under the GNU General Public License. Others have also developed alternative implementations of these Sun technologies, such as the GNU Compiler for Java and GNU Class path.
Principles
There were five primary goals in the creation of the Java language:
- It should be "simple, object-oriented and familiar"
- It should be "robust and secure"
- It should be "architecture-neutral and portable"
- It should execute with "high performance"
- It should be "interpreted, threaded, and dynamic"
Versions
Major release versions of Java, along with their release dates:
- JDK 1.0 (October 1, 1992)
- JDK 1.1 (February 19, 1997)
- J2SE 1.2 (December 8, 1998)
- J2SE 1.3 (May 8, 2000)
- J2SE 1.4 (February 6, 2002)
- J2SE 5.0 (September 30, 2004)
- Java SE 6 (December 11, 2006)
- Java SE 7 (July 28, 2011)
Practices
Java platform
Main articles: Java (software platform) and Java Virtual Machine
One characteristic of Java is portability, which means that computer programs written in the Java language must run similarly on any hardware/operating-system platform. This is achieved by compiling the Java language code to an intermediate representation called Java byte code, instead of directly to platform-specific machine code. Java bytecode instructions are analogous to machine code, but are intended to be interpreted by a virtual machine (VM) written specifically for the host hardware. End-users commonly use a Java Runtime Environment (JRE) installed on their own machine for standalone Java applications, or in a Web browser for Java applets.
Standardized libraries provide a generic way to access host-specific features such as graphics, threading, and networking.
A major benefit of using bytecode is porting. However, the overhead of interpretation means that interpreted programs almost always run more slowly than programs compiled to native executables would. Just-in-Time (JIT) compilers were introduced from an early stage that compile bytecodes to machine code during run time
Implementations
See also: Free Java implementations
Oracle Corporation is the current owner of the official implementation of the Java SE platform, following their acquisition of Sun Micro systems on January 27, 2010. This implementation is based on the original implementation of Java by Sun. The Oracle implementation is available for Mac OS X, Windows and Solaris. Because Java lacks any formal standardization recognized by Ecma International, ISO/IEC, ANSI, or any other third-party standards organization, the Oracle implementation is the de facto standard.
The Oracle implementation are packaged into two different distributions. The Java Runtime Environment (JRE) which contains the parts of the Java SE platform required to run Java programs. This package is intended for end-users. The Java Development Kit (JDK), is intended for software developers and includes development tools such as the Java compiler, Java doc, Jar, and a debugger.
OpenJDK is another notable Java SE implementation that is licensed under the GPL. The implementation started when Sun began releasing the Java source code under the GPL. As of Java SE 7, OpenJDK is the official Java reference implementation.
The goal of Java is to make all implementations of Java compatible. Historically, Sun's trademark license for usage of the Java brand insists that all implementations be "compatible". This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support RMI or JNI and had added platform-specific features of their own. Sun sued in 1997, and in 2001 won a settlement of US$20 million, as well as a court order enforcing the terms of the license from Sun.As a result, Microsoft no longer ships Windows with Java.
Platform-independent Java is essential to Java EE, and an even more rigorous validation is required to certify an implementation. This environment enables portable server-side applications.
Performance
Main article: Java performance
Programs written in Java have a reputation for being slower and requiring more memory than those written in C.However, Java programs' execution speed improved significantly with the introduction of Just-in-time compilation in 1997/1998 for Java 1.1,the addition of language features supporting better code analysis (such as inner classes, the StringBuffer class, optional assertions, etc.), and optimizations in the Java Virtual Machine itself, such as Hot Spot becoming the default for Sun's JVM in 2000. As of February 2012, microbenchmarks show Java 7 is approximately 1.5 times slower than C.
Some platforms offer direct hardware support for Java; there are microcontrollers that can run Java in hardware instead of a software Java Virtual Machine, and ARM based processors can have hardware support for executing Java bytecode through their Jazelle optio
Automatic memory management
Java uses an automatic garbage collector to manage memory in the object life cycle. The programmer determines when objects are created, and the Java runtime is responsible for recovering the memory once objects are no longer in use. Once no references to an object remain, the unreachable memory becomes eligible to be freed automatically by the garbage collector. Something similar to a memory leak may still occur if a programmer's code holds a reference to an object that is no longer needed, typically when objects that are no longer needed are stored in containers that are still in use. If methods for a nonexistent object are called, a "null pointer exception" is thrown.
One of the ideas behind Java's automatic memory management model is that programmers can be spared the burden of having to perform manual memory management. In some languages, memory for the creation of objects is implicitly allocated on the stack, or explicitly allocated and deallocated from the heap. In the latter case the responsibility of managing memory resides with the programmer. If the program does not deallocate an object, a memory leak occurs. If the program attempts to access or deallocate memory that has already been deallocated, the result is undefined and difficult to predict, and the program is likely to become unstable and/or crash. This can be partially remedied by the use of smart pointers, but these add overhead and complexity. Note that garbage collection does not prevent "logical" memory leaks, i.e. those where the memory is still referenced but never used.
Garbage collection may happen at any time. Ideally, it will occur when a program is idle. It is guaranteed to be triggered if there is insufficient free memory on the heap to allocate a new object; this can cause a program to stall momentarily. Explicit memory management is not possible in Java.
Java does not support C/C++ style pointer arithmetic, where object addresses and unsigned integers (usually long integers) can be used interchangeably. This allows the garbage collector to relocate referenced objects and ensures type safety and security.
As in C++ and some other object-oriented languages, variables of Java's primitive data types are not objects. Values of primitive types are either stored directly in fields (for objects) or on the stack (for methods) rather than on the heap, as commonly true for objects (but see Escape analysis). This was a conscious decision by Java's designers for performance reasons. Because of this, Java was not considered to be a pure object-oriented programming language. However, as of Java 5.0, auto boxing enables programmers to proceed as if primitive types were instances of their wrapper class.
Java contains multiple types of garbage collectors. By default, HotSpot uses the Concurrent Mark Sweep collector, also known as the CMS Garbage Collector. However, there are also several other garbage collectors that can be used to manage the Heap. For 90% of applications in Java, the CMS Garbage Collector is good enough.
Syntax
Main article: Java syntax
The syntax of Java is largely derived from C++. Unlike C++, which combines the syntax for structured, generic, and object-oriented programming, Java was built almost exclusively as an object-oriented language. All code is written inside a class, and everything is an object, with the exception of the primitive data types (e.g. integers, floating-point numbers, boolean values, and characters), which are not classes for performance reasons.
Unlike C++, Java does not support operator overloading or multiple inheritance for classes. This simplifies the language and aids in preventing potential errors and anti-pattern design.
Java uses similar commenting methods to C++. There are three different styles of comments: a single line style marked with two slashes (
//), a multiple line style opened with /* and closed with */, and the Javadoc commenting style opened with /**and closed with */. The Javadoc style of commenting allows the user to run the Javadoc executable to compile documentation for the program.Applet
The import statements direct the Java compiler to include the javax.swing.JApplet and java.awt.Graphics classes in the compilation. The import statement allows these classes to be referenced in the source code using the simple class name (i.e. JApplet) instead of the fully qualified class name (i.e. javax.swing.JApplet).
The
Hello class extends (subclasses) the JApplet (Java Applet) class; the JApplet class provides the framework for the host application to display and control the lifecycle of the applet. The JApplet class is a JComponent (Java Graphical Component) which provides the applet with the capability to display a graphical user interface (GUI) and respond to user events.
The
Hello class overrides the paintComponent(Graphics) method (additionally indicated with the annotation, supported as of JDK 1.5, Override) inherited from the Container superclass to provide the code to display the applet. ThepaintComponent() method is passed a Graphics object that contains the graphic context used to display the applet. ThepaintComponent() method calls the graphic context drawString(String, int, int) method to display the "Hello, world!" string at a pixel offset of (65, 95) from the upper-left corner in the applet's display.An applet is placed in an HTML document using the <applet> HTML element. The applet tag has three attributes set:code="Hello" specifies the name of the JApplet class and width="200" height="200" sets the pixel width and height of the applet. Applets may also be embedded in HTML using either the object or embed element,although support for these elements by Web browsers is inconsistent.However, the applet tag is deprecated, so the object tag is preferred where supported.
The host application, typically a Web browser, instantiates the
Hello applet and creates an AppletContext for the applet. Once the applet has initialized itself, it is added to the AWT display hierarchy. The paintComponent() method is called by the AWT event dispatching thread whenever the display needs the applet to draw itself.Servlet
Main article: Java Servlet
Java Servlet technology provides Web developers with a simple, consistent mechanism for extending the functionality of a Web server and for accessing existing business systems. Servlets are server-side Java EE components that generate responses (typically HTML pages) to requests (typically HTTP requests) from clients. A servlet can almost be thought of as an applet that runs on the server side—without a face.
The
import statements direct the Java compiler to include all of the public classes and interfaces from the java.io andjavax.servlet packages in the compilation.
The Hello class extends the GenericServlet class; the GenericServlet class provides the interface for the serverto forward requests to the servlet and control the servlet's lifecycle.
The
Hello class overrides the service(ServletRequest, ServletResponse) method defined by the Servletinterface to provide the code for the service request handler. The service() method is passed: a ServletRequest object that contains the request from the client and a ServletResponse object used to create the response returned to the client. The service() method declares that it throws the exceptions ServletException and IOException if a problem prevents it from responding to the request.
The
setContentType(String) method in the response object is called to set the MIME content type of the returned data to "text/html". The getWriter() method in the response returns a PrintWriter object that is used to write the data that is sent to the client. The println(String) method is called to write the "Hello, world!" string to the response and then the close() method is called to close the print writer, which causes the data that has been written to the stream to be returned to the client.