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Java Virtual Machine (JVM) and just in time compilation
- 2. Java Architecture Various components of the Java architecture are: Java programming language Java class file Java Virtual Machine (JVM) Java Application Programming Interface (API)
- 3. Java Programming Language and class File Java programs are saved with an extension, .java. A .java file is compiled to generate the .class file, which contains the Bytecode. The JVM converts the Bytecode contained in the .class file to machine object code. The JVM needs to be implemented for each platform running on a different operating system.
- 4. Java Virtual Machine (JVM) The JVM forms the base for the Java platform and is convenient to use on various hardware-based platforms. Components of the JVM JVM for different platforms uses different techniques to execute the Bytecode. The major components of JVM are: Class loader Execution engine Just In Time (JIT) compiler
- 5. ClassLoader Subsystem Java's dynamic class loading functionality is handled by the ClassLoader subsystem. It loads, links. and initializes the class file when it refers to a class for the first time at runtime, not compile time. 1.1 Loading Classes will be loaded by this component. BootStrap ClassLoader, Extension ClassLoader, and Application ClassLoader are the three ClassLoaders that will help in achieving it. 1. BootStrap ClassLoader– This is the first classloader which is the super class of Extension classloader. It loads the rt.jar file which contains all class files of Java Standard Edition like java.lang package classes, java.net package classes, java.util package classes, java.io package classes, java.sql package classes etc. 2. Extension ClassLoader – Responsible for loading classes which are inside the ext folder (jrelib). 3. Application ClassLoader –Responsible for loading Application Level Classpath, path mentioned Environment Variable, etc.
- 6. 1.2 Linking 1. Verify – Bytecode verifier will verify whether the generated bytecode is proper or not if verification fails we will get the verification error. 2. Prepare – For all static variables memory will be allocated and assigned with default values. 3. Resolve – All symbolic memory references are replaced with the original references from Method Area. 1.3 Initialization This is the final phase of ClassLoading; here, all static variables will be assigned with the original values, and the static block will be executed.
- 7. Execution Engine The Java execution engine is the component of the JVM that runs the Bytecode one line after another. The execution engines implemented by different vendors use different techniques to run the Bytecode. The Java execution engine converts the Bytecode to the machine object code and runs it.
- 8. JIT Compiler The JIT compiler is used for compiling the Bytecode into executable code. The JVM runs the JIT compiled code without interpreting because the JIT-compiled code is in the machine code format. Running the JIT- compiled code is faster than running the interpreted code because it is compiled and does not require to be run, line after line.
- 9. Java Application Programming Interface (API) The Java API is a collection of software components that provide capabilities, such as GUI. The related classes and interfaces of the Java API are grouped into packages. The following figure shows how the Java API and the JVM forms the platform for the Java programs on top of the hardware:
- 10. Java Architecture Security Java architecture also consists of a few security features that make Java a secure programming language. The architecture consists of the following security features: Compiler level security Bytecode verifier Class loader Sandbox model
- 12. Compiler Level Security Java prevents errors that arise due to improper memory usage, reducing the compile- time errors. In addition, Java employs typecasting between data types that implies that when one type of data is assigned to another type of variable, an automatic type conversion takes place. Typecasting in Java ensures that there is no data loss in the result or output of a Java code.
- 13. Bytecode Verifier The Bytecode verifier checks the Bytecode before it runs on a computer. It ensures that the Bytecode does not violate access restrictions, such as read/write operations, and verifies that the Bytecode does not forge pointers. The Bytecode is verified in two phases: In the first phase, the verifier checks for the structure of the .class file. The second phase occurs when the Bytecode is run. The Bytecode verifier checks the validity of classes, variables, and methods used in a program.
- 14. Class Loader The class loader determines how and when an applet will use classes in a running Java environment. In a Java environment, there can be many class loaders and each class loader can create its own run-time environment. The class loader loads all the applets and their references.
- 15. Sandbox Model The sandbox model is implemented in the Java applets container, such as Web browsers. The sandbox model determines the limitations of Java applets that they can only access the resources of the host computer and cannot access the files on the local computer. Java applications do not employ this security model and are allowed to read from and write to files on the local computers.