Android Programming Tutorial 39 : JVM vs DVM In Android

JVM vs DVM:

 
                                                   Vs

1.Conceptually, there is little difference from an application level between a DVM and a JVM. Architecturally, there is a major difference between the  register-based DVM and the stack-based JVM.

Both use a VM code model. However, the DVM uses register-based opcodes

that are comparable to the register-based bytecode instructions that most of the target platforms already execute. This includes architectures such as those available from ARM and MIPS and the x86-compatible architectures from Intel,

AMD, and VIA Technologies.

Google developed Android and chose DVM for several reasons. First, there were licensing issues with most JVMs. Next, the DVM should be more efficient in terms of memory usage and performance on a register-based machine. DVM is also supposed to be more efficient when running multiple instances of the DVM. Applications are given their own instance. Hence, multiple active applications require multiple DVM instances. Like most Java implementations, the DVM has an automatic garbage collector.

2) The jvm architecture is stack-based whereas the dvm architecture is register-based. Stack-based machines require more instructions(i.e. larger instruction set) than register-based machines for the same task. On the other side, each instruction in the register-based machines are larger. 

3). Jvm will work based on byte code and the dvm will works based on optimized bytecode it is optimised for mobile platforms because mobile devices have low memory and low process that's why it is using the linux kernel.

4). When a Java virtual machine start running a program, it needs memory to store many things, including bytecodes and other information it extracts from loaded class files, objects the program instantiates, parameters to methods, return values, local variables, and intermediate results of computations.The Java virtual machine organizes the memory it requires to execute a program into several runtime data areas. 

Generally, stack-based machines must use instructions to load data on the stack and manipulate that data, and, thus, require more instructions than register machines to implement the same high level code, but the instructions in a register machine must encode the source and destination registers and, therefore, tend to be larger

This difference is primarily of importance to VM interpreters for whom opcode dispatch tends to be expensive and other factors are relevant for JIT Compilation.Being optimized for low memory requirements, Dalvik has some specific characteristics that differentiate it from other standard VMs>>>>> 

The VM was just slimmed down to use less space--->>Dalvik currently has no just-in-time-compiler (JIT), but Android 2.0 includes experimental source for one (disabled by default). The constant pool has been modified to use only32-bit indexes to simplify the interpreter. It uses its own bytecode, not Java bytecode

5).DVM is Register based which is designed to run on low memory, uses its own byte code and runs .Dex file.

JVM is Stack based which uses java byte code and runs .class file having JIT. 

Java source code is compiled by the Java compiler into .class files. Then the dx (dexer) tool, part of the Android SDK processes the .class files into a file format called DEX that contains Dalvikbytecode. 

The dx tool eliminate all the redundant information that is present in the classes. In DEX all the classes of the application are packed into one file. DVM has been designed so that a device can run multiple instances of the VM efficiently. 

stack-based machines must use instructions to load data on the stack and manipulate that data, and, thus, require more instructions than register machines to implement the same high level code, but the instructions in a register machine must encode the source and destination registers and, therefore, tend to be larger. 

Dalvik Virtual machine (DVM) is Register Architecture, designed to run on low memory, uses its own byte code and runs .Dex file (Dalvik Executable File).

Java Virtual Machine (JVM) is Stack Architecture, uses java byte code and runs .class file having JIT. 

Java  source code is compiled by the Java compiler into .class files. Then the dx (dexer) tool, part of the Android SDK processes the .class files into a proprietary file format called DEX that contains Dalvikbytecode. 

Difference between .class and .dex file format  ● In DEX all the classes of the application are packed into one file.  ● All the classes in the same DEX file share field, method, etc if they are same.  ● Classes from the same DEX file are loaded by the same class loader instance.  Click on the link below to view in graphic the difference between the two:  Difference between .class and .dex file The dx tool eliminate all the redundant information that is present in the classes. The initial loading and parsing procedures that happens again and again are eliminated.  In general, a Java .class file contains a number of different method signatures used in the code. These signatures are duplicated if referenced in different Java classes.In other words, each of these classes references to the methods that takes the same arguments and the same return type. So, each one of these classes will have to include the same method signatures and hence the efforts of parsing the files are duplicated. In addition to this, there is a large number of strings included that labels the internal bits of the class files.  In the dex conversion all of the separate constant pools are collapsed into a single shared constant pool. This way not only is redundant information eliminated but storage space to store the shared constant pool is also conserved. Please Send Your Comments To ambilpura.sunil@gmail.com Stay Tune For Next Tutorial...  JSON Parsing In Android: