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JVM in Context oop What’s a JVM Platform-independent execution environment A different JVM is required for each operating system Any JVM can run the same version of a Java program Converts Java bytecode into machine language and executes it. oop Platform independence Programs can run without change on any networked computer Simple administration Programs can run on embedded devices Printers, scanners… Networks as homogeneous execution environments Enables building distributed systems Reduce time and cost of development More potential customers oop JVM in Java Architecture Java’s architecture main technologies: The Java programming language Source files The Java class file format Compiled files The Java API Provide access to system resources The Java virtual machine Runs the class files oop The Java Programming Environment Compile time environment A.Java B.Java C.Java oop B.class A.class B.class C.class Java Virtual Machine Java Compiler A.class run time environment C.class Object class String class Same Program, Different Platforms oop Java Program Java Program Java Program Java Program Java Platform for Linux Java Platform for Win32 Java Platform for your Television Java Platform for your Toaster Linux Box PC running Windows XP Your Television Your Toaster The JVM Roles Load class files Both program and API files Only files from API that are actually needed by the program are loaded Execute bytecode Interpretation – one method bytecode at a time Just in time compiler – compiles bytecodes of a method to native machine code the first time the method is invoked, cache the native code. Adaptive optimizer – identifies the most heavily used areas, compile them to native code. oop Basic JVM Components The Java Virtual Machine Program Class files The Java API’s class files Class loader Execution engine Native methods invocation Host operating system oop The Java Class File The binary form for Java programs Represents a complete description of one Java class or interface Platform independent – bytecodes are the machine language of the JVM Not necessarily linked to the java language: Java Program in Java Lang. Program in other Lang. Compiler class files Java Compiler class files Program in Java Lang. oop Compiler Other Binary format The Java API Set of runtime libraries Provide access to system resources of the host computer A required component of any implementation of the Java platform Java Program Java Methods (Java API) Native methods (dynamic libraries) Host operating system oop JVM Major Subsystems and Memory Areas. oop The Class Loader Subsystem oop The class loader performs three main functions of JVM, namely: loading, linking and initialization. The linking process consists of three sub-tasks, namely, verification, preparation, and resolution, as shown by the following figure. These activities are performed in a strict order as shown by the figure. Class Loading Loading means reading the class file for a type, parsing it to get its information, and storing the information in the method area. For each type, the JVM stores reflection information and bytecodes in the method area. oop Linking : Verification Ensuring that binary representation of a class is structurally correct. Checks that each instruction will find a correctly formed stack and local variable array checks that the items on top of the stack, and in the local variables, are of the correct type for the instruction None of the following are allowed: Type errors oop Operand stack overflow or underflow Access control violations (e.g., private fields and methods) Reading of uninitialized variables Use of uninitialized objects Wild jumps Preparation oop Allocating memory for the class (i.e static) variables and sets them to default initial values. Class variables are not initialized to their proper initial values until the initialization phase - no java code is executed until initialization. The default values for the various types are shown below: Resolution Replacing symbolic names for types, fields and methods used by a loaded type with their actual references. Symbolic references are resolved into direct references by searching through the method area to locate the referenced entity. oop Class Initialization Setting class variables to their proper initial values initial values desired by the programmer. Initialization of a class consists of two steps: Initializing its direct superclass (if any and if not already initialized) Executing its own initialization statements The above imply that, the first class that gets initialized is Object. oop JVM Memory Areas oop Each instance of the JVM has one method area, one heap, and one or more stacks - one for each thread. When JVM loads a class file, it puts its information in the method area. As the program runs, all objects instantiated are stored in the heap. The stack area is used to store activation records as a program runs. The Method Area Information about types Name, super class, modifiers… Constant pool Constants used by the type Fields and methods information Class (static) variables Size is not fixed Can be garbage collected Unreferenced classes can be unloaded oop JVM Stack vs. Heap Java stack Used for method activation frames One for each thread No restrictions in its layout Checked for overflow Java native stacks Native methods are used like other JVM methods, except they are implemented in some other language (e.g. C) Can be used for things that cannot be handled well in Java, e.g., integrating legacy code written in other languages Native stacks are used to keep track of state of native methods Behave like the stack in C Java heap Used for allocating objects Managed by a garbage collector Each object is associated with a class stored in the method area oop Activation Frames One is pushed on the Java stack for each method invocation The frame is popped when the invocation finishes A frame contains Local variables array Organized as an array Contains method’s parameters and local variables Size is determined at compile-time An operand stack Used for passing method arguments and results Only the top (few) words are accessed by any instruction Size is determined at compile-time oop Architectural Tradeoffs Program execution performance Interpreted languages are slower than running compiled native code JIT improves performance Memory management control Garbage collection improve robustness but adds a level of uncertainty to performance Platform independence Supports only the “lowest common denominator” capabilities oop
