Java and the JVM
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Java programs run on the Java Virtual Machine (JVM). The JVM provides a runtime environment that executes Java bytecode. Key aspects of Java include its use of object-oriented programming, garbage collection, and strong typing. Popular integrated development environments for Java include Eclipse and IntelliJ IDEA.
Java and the JVM
- 1. Java and the JVM Manish Pandit IGN Engineering
- 2. Java Echosystem • Java as a programming language • Java as runtime platform/virtual machine • Java libraries (collections, database drivers..)
- 3. Java the programming language • Object Oriented • Multi-threaded, Concurrent • Strongly typed • Garbage collection • No multiple inheritance • 2nd most popular language after C at 16%
- 4. IDEs • Please, no vim, emacs, pico etc. I completely get it that you’re rock awesome when you code on the command line with these tools. • The IDEs were built to make you productive. • I use Eclipse, the team uses IntelliJ IDEA. Both are good. • Use anything as long as it’s a real IDE
- 5. Code Organization • Package Declaration • Imports • Class or Interface Declaration • Members – Class variables – Methods
- 6. Imports • To use classes from a binary distribution or source • Can be wildcarded (discouraged) • Auto cleanup – Eclipse Ctl-Shift-O • Unused Imports
- 7. Packages • To namespace the Class or Interface • Act as modules containing groups of Classes or Interfaces • Convention – dot-separated – com., net., sf., org. • Can contain subpackages • Packaging can impact visibility if default scope is used.
- 8. Classes • Have to have the same file name as the public class • May or may not be a part of a package • Can be abstract or concrete and/or final • Acts as a template to create Objects
- 9. Classes • Have a constructor – Can be private (for singletons) – Default no-args constructor – Objects created by using new <T> varName = new <? extends T>(args); String myName = new String(“Manish”);
- 10. Control Structures • if-else • for loops • ternary operator (: ?) • while loops • switch/case • try/catch/finally • break/continue for loop control
- 11. Using this • this provides a reference to the current instance • Static members cannot use this, as they do not have instances (think Class, not Object)
- 12. Typed Collections and Classes • Introduced in Java 1.5 • Add strong typing via declaration, so the compile time checks can be performed • Syntax: Collection myCollection = new ArrayList<String>();
- 13. Annotations • Declarative programming – @SuppressWarnings – @Override – @Deprecated • Custom annotations – An annotation is an @interface
- 14. Threading • Two ways – Implement Runnable Interface – Extend Thread class • In both cases, you put the implementation in a method called run() • A thread is started by instantiating the Thread and calling start() on it. Never call run() directly.
- 15. Concurrent Code • synchornized method – Makes a method thread safe – You cannot synchronize a constructor • synchronized block – You can acquire a lock on an object, and write your code as synchronized(lock) {…} – All synchronized methods of a class use the same lock if you use synchronize(this) so be careful!
- 17. Dependency Injection • Is used to specify dependencies at runtime, which get injected (instantiated, associated) on class initialization. • Popular Frameworks – Spring DI – Google guice • Declared via configuration, or annotations
- 18. Maven • Maven (and Ant, and Gradle..) are build tools used to manage (large) java projects • Helps manage dependencies declaratively • Rich set of plugins to run tests, generate javadocs, build sites and artifacts • Everything comes together in pom.xml file.
- 19. JVM • A very efficient, tuned virtual machine • Runs bytecode • Runtime garbage collection • Supports monitoring via JMX • Concurrent • Target VM for Groovy, Scala and Clojure
- 20. JVM Memory Model • Heap – Where the instance, static variables and Objects go – Shared across all threads in the VM – Automatically garbage collected • Stack – Where the methods and local variables go – Every thread gets its own stack
- 21. JVM Performance Management • Heap and stack sizes • GC algorithm tuning • Heap monitoring (jprofiler) • Deadlocks • JMX (jconsole)
- 22. Java Libraries • Utilities – Apache Commons – JodaTime – Google collections • Web Frameworks – JSF – Struts • Application Frameworks – Spring – Play! Framework
- 23. Common java packages Package Description Java.lang.* Has the core classes like threading, runtime, Data types.. Java.net.* Has networking classes and adapters (think URL) Java.util.* Hash Collections, Calendar, Time/Locale classes Java.io.* Has Stream and Buffer handling classes for I/O Javax.swing.* Thick Client (UI) classes
- 24. Further reading • Javadocs • Java Language Specification • Wikipedia