Java virtual machine

Garbage collectors

Garbage collectors available in Java OpenJDKs virtual machine (JVM) include:

• Serial
• Parallel
• CMS (Concurrent Mark Sweep)
• G1 (Garbage-First)
• ZGC (Z Garbage Collector)
• Epsilon
• Shenandoah
• GenZGC (Generational ZGC)
• GenShen (Generational Shenandoah)
• IBM Metronome (only in IBM OpenJDK)
• SAP (only in SAP OpenJDK)
• Azul C4 (Continuously Concurrent Compacting Collector)^[8] (only in Azul Systems OpenJDK)

Java's design philosophy revolves around the assumption of a garbage collector. Unlike languages such as C++ and Rust, deterministic memory management through a delete keyword (as in C++) is not possible. Even introducing such a feature is not possible, due to a lack of ownership, aside from using sun.misc.Unsafe or through java.lang.foreign to allocate/deallocate memory outside the Java heap.^[9][10] Due to Java primarily using heap-based allocation, objects are stored as references, and deletion would result in dangling pointers.

Foo a = new Foo();
Foo b = a;

delete a;
System.out.println(b);

JVM languages

A JVM language is any language with functions that can be expressed in a valid class file, which can be hosted by a JVM. A class file contains JVM instructions (JVM bytecode), a symbol table, and other ancillary information. The class file format is the hardware- and operating system-independent binary format used to represent compiled classes and interfaces.^[12]

There are several JVM languages, both older established languages ported to JVM, and much newer from-scratch languages. JRuby and Jython may be the best known ports of older languages, i.e., Ruby and Python respectively. Of the new from-scratch languages created to compile to Java bytecode, Clojure, Groovy, Scala and Kotlin may be the most popular. A notable feature of the JVM languages is their language interoperability, they are compatible with each other, so that, for example, Scala libraries can be used with Java programs and vice versa.^[13]

Java 7 JVM implements JSR 292: Supporting Dynamically Typed Languages^[14] on the Java Platform, a new feature which supports dynamically typed languages in the JVM. This feature is developed within the Da Vinci Machine project whose mission is to extend the JVM so that it supports languages other than Java.^[15][16]

Class loader

One of the organizational units of JVM byte code is a class. A class loader implementation must be able to recognize and load anything that conforms to the Java class file format. Any implementation is free to recognize other binary forms besides class files, but it must recognize class files.

The class loader performs three basic activities in this strict order:

1. Loading: finds and imports the binary data for a type
2. Linking: performs verification, preparation, and (optionally) resolution
   • Verification: ensures the correctness of the imported type
   • Preparation: allocates memory for class variables and initializing the memory to default values
   • Resolution: transforms symbolic references from the type into direct references.
3. Initialization: invokes Java code that initializes class variables to their proper starting values.

In general, there are three types of class loader: bootstrap class loader, extension class loader and System / Application class loader.

Every Java virtual machine implementation must have a bootstrap class loader that is capable of loading trusted classes, as well as an extension class loader or application class loader. The Java virtual machine specification does not specify how a class loader should locate classes.

Bytecode instructions

The JVM has instructions for the following groups of tasks:

The aim is binary compatibility. Each particular host operating system needs its own implementation of the JVM and runtime. These JVMs interpret the bytecode semantically the same way, but the actual implementation may be different. More complex than just emulating bytecode is compatibly and efficiently implementing the Java core API that must be mapped to each host operating system.

These instructions operate on a set of common abstracted data types rather the native data types of any specific instruction set architecture.

Bytecode verifier

A basic philosophy of Java is that it is inherently safe from the standpoint that no user program can crash the host machine or otherwise interfere inappropriately with other operations on the host machine, and that it is possible to protect certain methods and data structures belonging to trusted code from access or corruption by untrusted code executing within the same JVM. Furthermore, common programmer errors that often led to data corruption or unpredictable behavior such as accessing off the end of an array or using an uninitialized pointer are not allowed to occur. Several features of Java combine to provide this safety, including the class model, the garbage-collected heap, and the verifier.

The JVM verifies all bytecode before it is executed. This verification consists primarily of three types of checks:

• Branches are always to valid locations
• Data is always initialized and references are always type-safe
• Access to private or package private data and methods is rigidly controlled

The first two of these checks take place primarily during the verification step that occurs when a class is loaded and made eligible for use. The third is primarily performed dynamically, when data items or methods of a class are first accessed by another class.

The verifier permits only some bytecode sequences in valid programs, e.g. a jump (branch) instruction can only target an instruction within the same method. Furthermore, the verifier ensures that any given instruction operates on a fixed stack location,^[17] allowing the JIT compiler to transform stack accesses into fixed register accesses. Because of this, that the JVM is a stack architecture does not imply a speed penalty for emulation on register-based architectures when using a JIT compiler. In the face of the code-verified JVM architecture, it makes no difference to a JIT compiler whether it gets named imaginary registers or imaginary stack positions that must be allocated to the target architecture's registers. In fact, code verification makes the JVM different from a classic stack architecture, of which efficient emulation with a JIT compiler is more complicated and typically carried out by a slower interpreter. Additionally, the Interpreter used by the default JVM is a special type known as a Template Interpreter, which translates bytecode directly to native, register based machine language rather than emulate a stack like a typical interpreter.^[18] In many aspects the HotSpot Interpreter can be considered a JIT compiler rather than a true interpreter, meaning the stack architecture that the bytecode targets is not actually used in the implementation, but merely a specification for the intermediate representation that can well be implemented in a register based architecture. Another instance of a stack architecture being merely a specification and implemented in a register based virtual machine is the Common Language Runtime.^[19]

The original specification for the bytecode verifier used natural language that was incomplete or incorrect in some respects. A number of attempts have been made to specify the JVM as a formal system. By doing this, the security of current JVM implementations can more thoroughly be analyzed, and potential security exploits prevented. It will also be possible to optimize the JVM by skipping unnecessary safety checks, if the application being run is proven to be safe.^[20]

Bytecode interpreter and Just-in-Time compiler

For each hardware architecture a different Java bytecode interpreter is needed. When a computer has a Java bytecode interpreter, it can run any Java bytecode program, and the same program can be run on any computer that has such an interpreter.

When Java bytecode is executed by an interpreter, the execution will always be slower than the execution of the same program compiled into native machine language. This problem is mitigated by just-in-time (JIT) compilers for executing Java bytecode. A JIT compiler may translate Java bytecode into native machine language while executing the program. The translated parts of the program can then be executed much more quickly than they could be interpreted. This technique gets applied to those parts of a program frequently executed. This way a JIT compiler can significantly speed up the overall execution time.

There is no necessary connection between the Java programming language and Java bytecode. A program written in Java can be compiled directly into the machine language of a real computer and programs written in other languages than Java can be compiled into Java bytecode.

Java bytecode is intended to be platform-independent and secure.^[22] Some JVM implementations do not include an interpreter, but consist only of a just-in-time compiler.^[23]

Java Native Interface

The Java Native Interface (or Native Method Interface) is a foreign function interface (non-Java) programming framework which enables Java code running in a Java virtual machine (JVM) to call and be called by native applications (programs specific to a hardware and operating system platform) and libraries written in other languages such as C, C++ and assembly.

Java 22 introduces the Foreign Function and Memory API, which can be seen as the successor to Java Native Interface.

Native methods are enabled by JNI to handle situations when an application cannot be written entirely in the Java programming language, e.g. when the standard Java class library does not support the platform-specific features or program library.

The JNI framework lets a native method use Java objects in the same way that Java code uses these objects. A native method can create Java objects and then inspect and use these objects to perform its tasks. A native method can also inspect and use objects created by Java application code.

JNI also allows direct access to assembly code, without even going through a C bridge.[3] Accessing Java applications from assembly is possible in the same way.

JavaScript JVMs and interpreters

Since May 2016, JavaPoly allows users to import unmodified Java libraries, and invoke them directly from JavaScript. JavaPoly allows websites to use unmodified Java libraries, even if the user does not have Java installed on their computer.^[26]

Transpilation to JavaScript

With the continuing improvements in JavaScript execution speed, combined with the increased use of mobile devices whose web browsers do not implement support for plugins, there are efforts to target those users through transpilation to JavaScript. It is possible to either transpile the source code or JVM bytecode to JavaScript.

Compiling the JVM bytecode, which is universal across JVM languages, allows building upon the language's existing compiler to bytecode. The main JVM bytecode to JavaScript transpilers are TeaVM,^[27] the compiler contained in Dragome Web SDK,^[28] Bck2Brwsr,^[29] and j2js-compiler.^[30]

Leading transpilers from JVM languages to JavaScript include the Java-to-JavaScript transpiler contained in Google Web Toolkit, J2CL,^[31] Clojurescript (Clojure), GrooScript (Apache Groovy), Scala.js (Scala) and others.^[32]