MicroEJ Core Engine

The MicroEJ Core Engine (also called the platform engine) and its components represent the core of the platform. It is used to compile and execute at runtime the MicroEJ Application code.

Functional Description

The following diagram shows the overall process. The first two steps are performed within the MicroEJ Workbench. The remaining steps are performed within the C IDE.

MicroEJ Core Engine Flow

Step 1 consists in writing a MicroEJ Application against a set of Foundation Libraries available in the platform.
Step 2 consists in compiling the MicroEJ Application code and the required libraries in an ELF library, using the SOAR.
Step 3 consists in linking the previous ELF file with the MicroEJ Core Engine library and a third-party BSP (OS, drivers, etc.). This step may require a third-party linker provided by a C toolchain.

Architecture

The MicroEJ Core Engine and its components have been compiled for one specific CPU architecture and for use with a specific C compiler.

The architecture of the platform engine is called green thread architecture, it runs in a single RTOS task. Its behavior consists in scheduling MicroEJ threads. The scheduler implements a priority preemptive scheduling policy with round robin for the MicroEJ threads with the same priority. In the following explanations the term “RTOS task” refers to the tasks scheduled by the underlying OS; and the term “MicroEJ thread” refers to the Java threads scheduled by the MicroEJ Core Engine.

A Green Threads Architecture Example

The activity of the platform is defined by the MicroEJ Application. When the MicroEJ Application is blocked (when all MicroEJ threads are sleeping), the platform sleeps entirely: The RTOS task that runs the platform sleeps.

The platform is responsible for providing the time to the MicroEJ world: the precision is 1 millisecond.

Capabilities

MicroEJ Core Engine defines 3 exclusive capabilities:

Mono-sandbox : capability to produce a monolithic firmware (default one).
Multi-Sandbox : capability to produce a extensible firmware on which new applications can be dynamically installed. See section Multi-Sandbox.
Tiny application : capability to produce a compacted firmware (optimized for size). See section Tiny application.

All MicroEJ Core Engine capabilities may not be available on all architectures. Refer to section Supported MicroEJ Core Engine Capabilities by Architecture Matrix for more details.

Implementation

The platform implements the [SNI] specification. It is created and initialized with the C function SNI_createVM. Then it is started and executed in the current RTOS task by calling SNI_startVM. The function SNI_startVM returns when the MicroEJ Application exits. The function SNI_destroyVM handles the platform termination.

The file LLMJVM_impl.h that comes with the platform defines the API to be implemented. The file LLMJVM.h that comes with the platform defines platform-specific exit code constants. (See LLMJVM: MicroEJ Core Engine.)

Initialization

The Low Level MicroEJ Core Engine API deals with two objects: the structure that represents the platform, and the RTOS task that runs the platform. Two callbacks allow engineers to interact with the initialization of both objects:

LLMJVM_IMPL_initialize: Called once the structure representing the platform is initialized.
LLMJVM_IMPL_vmTaskStarted: Called when the platform starts its execution. This function is called within the RTOS task of the platform.

Scheduling

To support the green thread round-robin policy, the platform assumes there is an RTOS timer or some other mechanism that counts (down) and fires a call-back when it reaches a specified value. The platform initializes the timer using the LLMJVM_IMPL_scheduleRequest function with one argument: the absolute time at which the timer should fire. When the timer fires, it must call the LLMJVM_schedule function, which tells the platform to execute a green thread context switch (which gives another MicroEJ thread a chance to run).

Idle Mode

When the platform has no activity to execute, it calls the LLMJVM_IMPL_idleVM function, which is assumed to put the RTOS task of the platform into a sleep state. LLMJVM_IMPL_wakeupVM is called to wake up the platform task. When the platform task really starts to execute again, it calls the LLMJVM_IMPL_ackWakeup function to acknowledge the restart of its activity.

Time

The platform defines two times:

the application time: The difference, measured in milliseconds, between the current time and midnight, January 1, 1970, UTC.
the system time: The time since the start of the device. This time is independent of any user considerations, and cannot be set.

The platform relies on the following C functions to provide those times to the MicroEJ world:

LLMJVM_IMPL_getCurrentTime: Depending on the parameter (true / false) must return the application time or the system time. This function is called by the MicroEJ method System.currentTimeMillis(). It is also used by the platform scheduler, and should be implemented efficiently.
LLMJVM_IMPL_getTimeNanos: must return the system time in nanoseconds.
LLMJVM_IMPL_setApplicationTime: must set the difference between the current time and midnight, January 1, 1970, UTC.

Example

The following example shows how to create and launch the MicroEJ Core Engine from the C world. This function (mjvm_main) should be called from a dedicated RTOS task.

#include <stdio.h>
#include "mjvm_main.h"
#include "LLMJVM.h"
#include "sni.h"

void mjvm_main(void)
{
    void* vm;
    int32_t err;
    int32_t exitcode;

    // create VM
    vm = SNI_createVM();

    if(vm == NULL)
    {
        printf("VM initialization error.\n");
    }
    else
    {
        printf("VM START\n");
        err = SNI_startVM(vm, 0, NULL);

        if(err < 0)
        {
            // Error occurred
            if(err == LLMJVM_E_EVAL_LIMIT)
            {
                printf("Evaluation limits reached.\n");
            }
            else
            {
                printf("VM execution error (err = %d).\n", err);
            }
        }
        else
        {
            // VM execution ends normally
            exitcode = SNI_getExitCode(vm);
            printf("VM END (exit code = %d)\n", exitcode);
        }

        // delete VM
        SNI_destroyVM(vm);
    }
}

Debugging

The internal MicroEJ Core Engine function called LLMJVM_dump allows you to dump the state of all MicroEJ threads: name, priority, stack trace, etc. This function can be called at any time and from an interrupt routine (for instance from a button interrupt).

This is an example of a dump:

============ VM Dump ============
2 java threads
---------------------------------
Java Thread[3]
name="SYSINpmp" prio=5 state=WAITING

java/lang/Thread:
    at com/is2t/microbsp/microui/natives/NSystemInputPump.@134261800
 [0x0800AC32]
    at com/is2t/microbsp/microui/io/SystemInputPump.@134265968
 [0x0800BC80]
    at ej/microui/Pump.@134261696
 [0x0800ABCC]
    at ej/microui/Pump.@134265872
 [0x0800BC24]
    at java/lang/Thread.@134273964
 [0x0800DBC4]
    at java/lang/Thread.@134273784
 [0x0800DB04]
    at java/lang/Thread.@134273892
 [0x0800DB6F]
---------------------------------
Java Thread[2]
name="DISPLpmp" prio=5 state=WAITING

java/lang/Thread:
    at java/lang/Object.@134256392
 [0x08009719]
    at ej/microui/FIFOPump.@134259824
 [0x0800A48E]
    at ej/microui/io/DisplayPump.134263016
 [0x0800B0F8]
    at ej/microui/Pump.@134261696
 [0x0800ABCC]
    at ej/microui/Pump.@134265872
 [0x0800BC24]
    at ej/microui/io/DisplayPump.@134262868
 [0x0800B064]
    at java/lang/Thread.@134273964
 [0x0800DBC4]
    at java/lang/Thread.@134273784
 [0x0800DB04]
    at java/lang/Thread.@134273892
 [0x0800DB6F]
=================================

See Stack Trace Reader for additional info related to working with VM dumps.

Generic Output

The System.err stream is connected to the System.out print stream. See below for how to configure the destination of these streams.

Link

Several sections are defined by the MicroEJ Core Engine. Each section must be linked by the third-party linker.

Linker Sections
Section name	Aim	Location	Alignment (in bytes)
`.bss.features.installed`	Resident applications statics	RW	4
`.bss.soar`	Application static	RW	8
`.bss.vm.stacks.java`	Application threads stack blocks	RW	8
`ICETEA_HEAP`	MicroEJ Core Engine internal heap	Internal RW	8
`_java_heap`	Application heap	RW	4
`_java_immortals`	Application immortal heap	RW	4
`.rodata.resources`	Application resources	RO	16
`.rodata.soar.features`	Resident applications code and resources	RO	4
`.shieldedplug`	Shielded Plug data	RO	4
`.text.soar`	Application and library code	RO	16

Dependencies

The MicroEJ Core Engine requires an implementation of its low level APIs in order to run. Refer to the chapter Implementation for more information.

Installation

The MicroEJ Core Engine and its components are mandatory. In the platform configuration file, check Multi Applications to install the MicroEJ Core Engine in “Multi-Sandbox” mode. Otherwise, the “Single application” mode is installed.

Use

A MicroEJ classpath variable named EDC-1.2 is available, according to the selected foundation core library. This MicroEJ classpath variable is always required in the build path of a MicroEJ project; and all others libraries depend on it. This library provides a set of options. Refer to the chapter Application Options which lists all available options.

Another classpath variable named BON-1.2 is available. This variable must be added to the build path of the MicroEJ Application project in order to access the [BON] library.