Implement a Blocking Java Native Method with SNI

This tutorial describes the good practices to follow when implementing a blocking native method in C. A native method is a method declared in Java with the native keyword and implemented in C using the Simple Native Interface (SNI).

Intended Audience

The audience for this document is Platform developers who want to implement Abstraction Layer interfaces.


The following document assumes the reader already has a setup ready to run a MicroEJ Standalone Application on a target device.

The following document also assumes the reader is familiar with the Simple Native Interface (SNI) mechanism. If not, the CallingCFromJava GitHub example shows the minimum steps required to create a Java program that makes a call to a C function via SNI.


The MicroEJ Core Engine implements a green thread architecture with all the Java threads executed within one single RTOS/OS task. Thus, it embeds its own scheduler that controls the execution of the Java threads. With such an architecture, the MicroEJ Core Engine cannot preempt a Java thread that executes a native method. Therefore a blocking native method will prevent the execution of other Java threads. To mitigate the contention, a native method must explicitly yield its current use of the processor.

This tutorial will explain how to use SNI to implement a blocking Java native method without blocking the execution of other Java threads.


A MicroEJ Platform with (at least):

Example Code

Let’s start with a MicroEJ Standalone Application that calls a blocking Java method implemented in C.

The following example waits for a button event and prints the index of the pressed button.

The MicroEJ Application code:

package example;

public class NativeCCallExample {

    public static void main(String[] args) {

        while (true) {
            System.out.println("Waiting for a button event...");
            int buttonIndex = waitButton();
            System.out.println("Button pressed: " + buttonIndex);

    public static native int waitButton();

The C implementation of the waitButton() native has been written in pseudo-code. It should be adapted according to the BSP of the target board.

#include "semaphore.h"
#include "sni.h"

static int pressed_button_index;
static Semaphore button_semaphore;

void button_init(){
    button_semaphore = SemaphoreCreateBinary();

jint Java_example_NativeCCallExample_waitButton(){
    // Wait for a button event
    return pressed_button_index;

/** Interrupt request handler called when a button is pressed. */
int buttonIRQ(int button_index){
    pressed_button_index = button_index;

Application Behavior

In this example, the execution of the waitButton() native method will block until a button is pressed. In other words, while Java_example_NativeCCallExample_waitButton() has not returned, no other Java thread can be scheduled.

This is because the native function is called in the same RTOS/OS task as the Java application.

This schematic explains what is going on:


Implement a Non-Blocking Method

This section will explain how to update the example code to make a non-blocking method.

Here is a summary of what will be done in C:

  • Signal the MicroEJ Core Engine to suspend the current thread when the native function returns.
  • Remove the blocking operations from the native function so that it returns immediately.
  • Implement a callback function that returns the index of the pressed button.
  • Register this callback function in the MicroEJ Core Engine to call it when the Java thread is resumed.
  • Resume the Java thread when a button is pressed.

This schematic summarizes the steps described above:


Update the C Native Function Implementation

Step 1: Update the C Native Function

The Java_example_NativeCCallExample_waitButton() function will now suspend the current Java thread. It will also store the information required to resume it and return the index of the pressed button.

The SNI functions used in this example are defined in sni.h. See this header file for a more detailed description of the API.

  • Store the ID of the Java thread that called the function. This ID should be stored in a global variable. It is used to resume the Java thread when a button is pressed.

    java_thread_id = SNI_getCurrentJavaThreadID();
  • Signal the MicroEJ Core Engine to suspend the current Java thread and specify the callback function to be called when the thread is resumed. Let’s call the callback function waitButton_callback().

    SNI_suspendCurrentJavaThreadWithCallback(0, (SNI_callback)waitButton_callback, NULL);

The function SNI_suspendCurrentJavaThreadWithCallback() returns immediately. The current thread is actually suspended when the native function returns.

The value returned by the Java_example_NativeCCallExample_waitButton() doesn’t matter anymore. The callback function will be in charge of returning the value.

The updated Java_example_NativeCCallExample_waitButton() function should look like this:

static int32_t java_thread_id;

jint Java_example_NativeCCallExample_waitButton(){

    java_thread_id = SNI_getCurrentJavaThreadID();

    SNI_suspendCurrentJavaThreadWithCallback(0, (SNI_callback)waitButton_callback, NULL);

    return SNI_IGNORED_RETURNED_VALUE; // Returned value not used

Step 2: Update the Button Interrupt Function

The role of the button interrupt is now to resume the Java thread when a button event occurs. Update it this way:

int buttonIRQ(int button_index){
    SNI_resumeJavaThreadWithArg(java_thread_id, (void*)button_index);

The button’s index is passed to the function SNI_resumeJavaThreadWithArg() so that the callback retrieves it when the thread is resumed.

Step 3: Implement the Callback Function

The callback function must have the same signature as the SNI native (same parameters and return type): jint waitButton_callback().

The callback function is automatically called by the Java thread when it is resumed. Use the SNI_getCallbackArgs() function to retrieve the arguments that was previously given to the SNI_suspendCurrentJavaThreadWithCallback() or SNI_resumeJavaThreadWithArg() functions.

jint waitButton_callback()
    int button_index;
    SNI_getCallbackArgs(NULL, (void*)&button_index);
    return (jint)button_index; // Actual value returned to Java

Application Behavior

In this configuration, calling the native method waitButton() will still return only when a button is pressed, but it will not prevent other Java threads from being scheduled.