Java and the JVM

Martin Schöberl

Java and the JVM 2

Overview

History and Java featuresJava technologyThe Java languageA first look into the JVMDisassembling of .class files

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History of a Young Java

1992 Oak for a PDA on a SPARC (*7)1995 Official release as Java – Internet1997 picoJava – Sun’s Java processor1998 RTSJ specification start as JSR-011999 split into J2SE and J2EE2000 J2ME2002 RTSJ final release2002 first version of JOP ;-)

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Java features

Simple and object orientedLook and feel of CSimplified object model with single inheritance

PortabilityJava compiler generates bytecodesRuntime systems for various platformsSize and behavior of basic data types definedWrite once, run/debug anywhere

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Java features cont.

AvailabilityWindows, Linux, Solaris,…Embedded systemsCompiler and runtime are freeFree IDEs: Eclipse, Netbeans

LibraryRich class libraryPart of the definitionStandard GUI toolkit

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Java features cont.

Built-in model for concurrencyThreads at the language levelSynchronizationLibraries are thread-safe

SafetyNo Pointer!Extensive compile-time checkingRuntime checkingAutomatic memory management – GC

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Java system overview

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Java Technology

The Java programming languageThe library (JDK)The Java virtual machine (JVM)

Instruction set Binary formatVerification

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Java Primitive Data Typesboolean either true or falsechar 16-bit Unicode character (unsigned)byte 8-bit integer (signed)short 16-bit integer (signed)int 32-bit integer (signed)long 64-bit integer (signed)float 32-bit floating-point (IEEE 754-1985)double 64-bit floating-point (IEEE 754-1985)

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Objects

Everything belongs to an object (or a class)No global variables

Namespace for objectsSingle inheritanceInterfacesAllocated on the heapShared among threadsNo free() – garbage collector

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What is a Virtual Machine?

A virtual machine (VM) is an abstractcomputer architectureSoftware on top of a real hardwareCan run the same application on different machines where the VM is available

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The Java Virtual Machine

An abstract computing machine that executes bytecode programs

An instruction set and the meaning of those instructions – the bytecodesA binary format – the class file formatAn algorithm to verify the class file

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JVM cont.

Runtime environment for JavaImplementation NOT definedRuns Java .class filesHas to conform to Sun‘s specification

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Implementations of the JVM

InterpreterSimple, compactSlow

Just-in-time compilationState-of-the-art for desktop/serverToo resource consuming in embedded systems

Batch compilationHardware implementation

Our topic!

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JVM Data Typesreference Pointer to an object or arrayint 32-bit integer (signed)long 64-bit integer (signed)float 32-bit floating-point (IEEE 754-1985)double 64-bit floating-point (IEEE 754-1985)

No boolean, char, byte, and short typesStack contains only 32-bit and 64-bit dataConversion instructions

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Memory Areas for the JVM

Method areaClass descriptionCodeConstant pool

HeapObjects and ArraysShared by all threadsGarbage collected

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Memory Areas for the JVM

StackThread privateLogical stack that contains:

Invocation frameLocal variable areaOperand stack

Not necessary a single stackLocal variables and operand stack are accessed frequently

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JVM Instruction Set

32 (64) bit stack machineVariable length instruction setSimple to very complex instructionsSymbolic referencesOnly relative branches

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JVM Instruction Set

Load and storeArithmeticType conversionObject creation and manipulationOperand stack manipulationControl transferMethod invocation and return

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Dissassembling Java

Compilejavac Hello.java

Run java Hello

Dissassemblejavap -c Hello

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A Bytecode Example

public class X {

public static voidmain(String[] args) {

add(1, 2);}

public static intadd(int a, int b) {

return a+b;}

}

public static void main(java.lang.String[]);Code:0: iconst_11: iconst_2//Method add:(II)I2: invokestatic #2; 5: pop6: return

public static intadd(int,int);Code:0: iload_01: iload_12: iadd3: ireturn

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Coding: Avoiding garbage

System.out.println("Result = "+i);

getstatic #3; // Field System.out:Ljava/io/PrintStream;new #4; // class StringBufferdupinvokespecial #5; // StringBuffer."<init>":()Vldc #6; // String Result = invokevirtual #7; // StringBuffer.append:(LString;)LStringBufferiload_1invokevirtual #8; // StringBuffer.append:(I)LStringBuffer;invokevirtual #9; // StringBuffer.toString:()LString;invokevirtual #10;// PrintStream.println:(LString;)V

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Coding: Avoiding garbage

System.out.print("Result = ");System.out.println(i);

getstatic #3; //Field System.out:Ljava/io/PrintStream;ldc #4; //String Result = invokevirtual #5; //Method PrintStream.print:(LString;)Vgetstatic #3; //Field System.out:LPrintStream;iload_1invokevirtual #6; //Method PrintStream.println:(I)V

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Java for Embedded Systems?

+ Simpler than C/C+++ Safer than C/C+++ Threads are part of the language- Interpreting JVM is slow- JIT needs a lot of memory- GC and real-time?

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Summary Java/JVM

Java language definitionClass libraryThe Java virtual machine (JVM)

An instruction set – the bytecodesA binary format – the class fileAn algorithm to verify the class file

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Summary Java Features

Safe OO LanguageNo pointersType-safetyGarbage Collection

Built in model for concurrencyPlatform independentVery rich standard library

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More Information

JavaJames Gosling, Bill Joy, Guy Steele, and GiladBracha. The Java Language Specification, Addison-Wesley, 2000, JavaSpec.

JVMTim Lindholm and Frank Yellin. The Java Virtual Machine Specification. Addison-Wesley, 1999, JVMSpec.