Shielded Plug (SP)


The Shielded Plug [SP] provides data segregation with a clear publish-subscribe API. The data-sharing between modules uses the concept of shared memory blocks, with introspection. The database is made of blocks: chunks of RAM.

A Shielded Plug Between Two Application (Java/C) Modules.

A Shielded Plug Between Two Application (Java/C) Modules.

Functional Description

The usage of the Shielded Plug (SP) starts with the definition of a database. The implementation of the [SP] for the MicroEJ Platform uses an XML file description to describe the database; the syntax follows the one proposed by the [SP] specification.

Once this database is defined, it can be accessed within the MicroEJ Application or the C application. The [SP] Foundation Library is accessible from the classpath variable SP-2.0. This library contains the classes and methods to read and write data in the database. See also the Java documentation from the MicroEJ Workbench resources center (“Javadoc” menu). The C header file sp.h available in the MicroEJ Platform source/MICROJVM/include folder contains the C functions for accessing the database.

To embed the [SP] database in your binary file, the XML file description must be processed by the [SP] compiler. This compiler generates a binary file (.o) that will be linked to the overall application by the linker. It also generates two descriptions of the block ID constants, one in Java and one in C. These constants can be used by either the Java or the C application modules.

Shielded Plug Compiler

A MicroEJ tool is available to launch the [SP] compiler tool. The tool name is Shielded Plug Compiler. It outputs:

  • A description of the requested resources of the database as a binary file (.o) that will be linked to the overall application by the linker. It is an ELF format description that reserves both the necessary RAM and the necessary Flash memory for the Shielded Plug database.
  • Two descriptions, one in Java and one in C, of the block ID constants to be used by either Java or C application modules.
Shielded Plug Compiler Process Overview

Shielded Plug Compiler Process Overview


Below is an example of using a database [SP]. The code that publishes the data is written in C, and the code that receives the data is written in Java. The data is transferred using two memory blocks. TEMP is a scalar value, THERMOSTAT is a boolean.

Database Description

The database is described as follows:

    <database name="Forecast" id="0" immutable="true" version="1.0.0">
        <block id="1" name="TEMP" length="4" maxTasks="1"/>
        <block id="2" name="THERMOSTAT" length="4" maxTasks="1"/>

Java Code

From the database description we can create an interface.

public interface Forecast {
   public static final int ID = 0;
   public static final int TEMP = 1;
   public static final int THERMOSTAT = 2;

Below is the task that reads the published temperature and controls the thermostat.

public void run(){
   ShieldedPlug database = ShieldedPlug.getDatabase(Forecast.ID);
   while (isRunning) {
      //reading the temperature every 30 seconds
      //and update thermostat status
      try {
         int temp = database.readInt(Forecast.TEMP);
         //update the thermostat status
         database.writeInt(Forecast.THERMOSTAT,temp>tempLimit ? 0 : 1);
      catch(EmptyBlockException e){
         print("Temperature not available");

C Code

Here is a C header that declares the constants defined in the XML description of the database.

#define Forecast_ID 0
#define Forecast_TEMP 1
#define Forecast_THERMOSTAT 2

Below, the code shows the publication of the temperature and thermostat controller task.

void temperaturePublication() {
   ShieldedPlug database = SP_getDatabase(Forecast_ID);
   int32_t temp = temperature();
   SP_write(database, Forecast_TEMP, &temp);

void thermostatTask(){
   int32_t thermostatOrder;
   ShieldedPlug database = SP_getDatabase(Forecast_ID);
      SP_waitFor(database, Forecast_THERMOSTAT);
      SP_read(database, Forecast_THERMOSTAT, &thermostatOrder);
      if(thermostatOrder == 0) {
      else {



The [SP] library and its relative tools are an optional feature of the platform. In the platform configuration file, check Java to C Interface > Shielded Plug to install the library and its relative tools.


A classpath variable named SP-2.0 is available, which must be added to the build path of the MicroEJ Application project in order to access the [SP] library.

This library provides a set of options. Refer to the chapter Application Options which lists all available options.