Stephan Erbs Korsholm

A Hardware Abstraction Layer in Java

Embedded systems use specialized hardware devices to interact with their environment, and since they have to be dependable, it is attractive to use a modern, type-safe programming language like Java to develop programs for them. Standard Java, as a platform-independent language, delegates access to devices, direct memory access, and interrupt handling to some underlying operating system or kernel, but in the embedded systems domain resources are scarce and a Java Virtual Machine (JVM) without an underlying middleware is an attractive architecture. The contribution of this article is a proposal for Java packages with hardware objects and interrupt handlers that interface to such a JVM. We provide implementations of the proposal directly in hardware, as extensions of standard interpreters, and finally with an operating system middleware. The latter solution is mainly seen as a migration path allowing Java programs to coexist with legacy system components. An important aspect of the proposal is that it is compatible with the Real-Time Specification for Java (RTSJ).

Hardware Objects for Java

Java, as a safe and platform independent language, avoids access to low-level I/O devices or direct memory access. In standard Java, low-level I/O it not a concern; it is handled by the operating system. However, in the embedded domain resources are scarce and a Java virtual machine (JVM) without an underlying middleware is an attractive architecture. When running the JVM on bare metal, we need access to I/O devices from Java; therefore we investigate a safe and efficient mechanism to represent I/O devices as first class Java objects, where device registers are represented by object fields. Access to those registers is safe as Javas type system regulates it. The access is also fast as it is directly performed by the bytecodes getfield and putfield. Hardware objects thus provide an object-oriented abstraction of low-level hardware devices. As a proof of concept, we have implemented hardware objects in three quite different JVMs: in the Java processor JOP, the JIT compiler CACAO, and in the interpreting embedded JVM SimpleRTJ.

Safety-critical Java for low-end embedded platforms

We present an implementation of the Safety-Critical Java profile (SCJ), targeted for low-end embedded platforms with as little as 16 kB RAM and 256 kB flash. The distinctive features of the implementation are a combination of a lean Java virtual machine (HVM), with a bare metal kernel implementing hardware objects, first level interrupt handlers, and native variables, and an infrastructure written in Java which is minimized through program specialization. The HVM allows the implementation to be easily ported to embedded platforms which have a C compiler as part of the development environment; the bare metal approach eliminates the need for a resource consuming operating system or C-library; the program specialization means that the infrastructure for the SCJ profile is optimized for a particular application keeping only the code and data the application needs. The SCJ implementation is evaluated with a known benchmark and shown to reduce this to a size where it can execute on a minimal configuration.

HVMTP: A Time Predictable and Portable Java Virtual Machine for Hard Real-Time Embedded Systems

We present HVMTP, a time predictable and portable Java Virtual Machine (JVM) implementation with applications in resource-constrained, hard real-time embedded systems, which implements the Safety Critical Java (SCJ) Level 1 specification. Time predictability is achieved by a combination of time predictable algorithms, exploiting the programming model of the SCJ profile, and harnessing static knowledge of the hosted SCJ system. This paper presents HVMTP in terms of its design and capabilities, and demonstrates how a complete timing model of the JVM represented as a Network of Timed Automata can be obtained using the tool TetaSARTSJVM. The timing model readily integrates with the rest of the TetaSARTS tool-set for temporal verification of SCJ systems. We will also show how a complete timing scheme in terms of safe Worst Case Execution Times and Best Case Execution Times of the Java Bytecodes can be derived from the model.

Interrupt Handlers in Java

An important part of implementing device drivers is to control the interrupt facilities of the hardware platform and to program interrupt handlers. Current methods for handling interrupts in Java use a server thread waiting for the VM to signal an interrupt occurrence. It means that the interrupt is handled at a later time, which has some disadvantages. We present constructs that allow interrupts to be handled directly and not at a later point decided by a scheduler. A desirable feature of our approach is that we do not require a native middelware layer but can handle interrupts entirely with Java code. We have implemented our approach using an interpreter and a Java processor, and give an example demonstrating its use.

A real-time Java tool chain for resource constrained platforms

The Java programming language was originally developed for embedded systems, but the resource requirements of previous and current Java implementations – especially memory consumption – tend to exclude them from being used on a significant class of resource constrained embedded platforms. The contribution of this paper is an architecture and implementation of a Java execution stack for resource constrained embedded platforms with a few kB of RAM and flash memory. The resource requirements of the presented architecture has been reduced significantly through all the layers of the architecture by integrating the following: (1) a lean virtual machine without any external dependencies on POSIX‐like libraries or other OS functionalities; (2) a hardware abstraction layer, implemented almost entirely in Java through the use of hardware objects, first level interrupt handlers, and native variables; and (3) an implementation of the Safety‐Critical Java profile Level 0 and 1 for hard real‐time applications. All Java components of a given application are minimized through program specialization, and because (2) and (3) are written in Java, the program specialization has a significant impact on the resulting program size. An evaluation of the presented solution shows that the miniCDj benchmark gets reduced to a size where it can run on resource constrained platforms. Copyright

HVMTP: A time predictable and portable java virtual machine for hard real‐time embedded systems

We present HVMTP, a time predictable and portable Java virtual machine (JVM) implementation with applications in resource‐constrained, hard real‐time embedded systems, which implements all levels of the safety critical Java (SCJ) specification. Time predictability is achieved by a combination of time‐predictable algorithms, exploiting the programming model of the SCJ profile and harnessing static knowledge of the hosted SCJ system. This paper presents HVMTP in terms of its design and capabilities and demonstrates how a complete timing model of the JVM represented as a network of timed automata can be obtained using the tool TETASARTSJVM. The timing model readily integrates with the rest of the TETASARTS tool set for temporal verification of SCJ systems. We will also show how a complete timing scheme in terms of safe worst‐case execution times and best‐case execution times of the Java bytecodes can be derived from the model. Furthermore, we take a first look at how to support the new Java 8 language feature of Lambda expressions in a SCJ context – we look in particular at how the invokedynamic bytecode can be implemented in a time‐predictable way and integrated in HVMTP. Copyright

Flash memory in embedded Java programs

This paper introduces a Java execution environment with the capability for storing constant heap data in Flash, thus saving valuable RAM. The extension is motivated by the structure of three industrial applications which demonstrate the need for storing constant data in Flash on small embedded devices. The paper introduces the concept of host initialization of constant data to prepare a Flash image of constant data that can be kept outside the heap during runtime. The concept is implemented in an interpreter based Java execution environment.

Safety-critical Java for embedded systems

This paper presents the motivation for and outcomes of an engineering research project on certifiable Java for embedded systems. The project supports the upcoming standard for safety‐critical Java, which defines a subset of Java and libraries aiming for development of high criticality systems. The outcome of this project include prototype safety‐critical Java implementations, a time‐predictable Java processor, analysis tools for memory safety, and example applications to explore the usability of safety‐critical Java for this application area. The text summarizes developments and key contributions and concludes with the lessons learned. Copyright

Conformance test development with the Java modeling language

In order to claim conformance with a Java Specification Request, a Java implementation has to pass all tests in an associated Technology Compatibility Kit. This paper presents a model‐based development of a Technology Compatibility Kit test suite and a test execution tool for the draft safety‐critical Java profile specification. The Java Modeling Language is used to model conformance constraints for the profile. Java Modeling Language annotations define contracts for classes and interfaces. The annotations are translated by a tool into runtime assertion checks. Hereby, the design and elaboration of the concrete test cases are simplified, because the expected results are derived from contracts and thus do not need to be provided explicitly. Bottom‐up testing is applied for testing methods of the safety‐critical Java classes, whereas top‐down testing is applied for testing global properties, such as protocols, memory management, and real‐time properties, including scheduling. The tests are executed using a simplified version of JUnit, which makes the test suite executable on resource‐constrained platforms. Copyright