android实现真随机数生成器（物理随机）

原理：通过设备内置传感器生成随机散列字串，再通过取余等操作获取最终数字，这种方法不同于传统的伪随机数生成器，而是依赖于硬件，需要硬件支持。

以下模块实现每隔0.5秒随机生成数字0和1。

private SensorManager sm;
    private int eventCount;
    private int flagCount;
    private StringBuffer mKeyBuffer = new StringBuffer();
    private boolean isRunning;
    private List numbers = new ArrayList<>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_sensor);

        sm = (SensorManager) getApplicationContext().getSystemService(Service.SENSOR_SERVICE);

        List sensors = sm.getSensorList(Sensor.TYPE_ALL);
        for (Sensor sensor : sensors) {
            sm.registerListener(this, sensor, SensorManager.SENSOR_DELAY_FASTEST);
        }

        isRunning = true;

        //final StringBuffer values = new StringBuffer();

        new Thread() {
            @Override
            public void run() {
                super.run();
                Dbg.print("====start====");
                while (isRunning) {
                    try {
                        Thread.sleep(500); //每隔500ms生成一个数
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    /*if (eventCount >= 100000) {
                        break;
                    }*/
                    if (mKeyBuffer.length() == 0) {
                        continue;
                    }
                    String source = mKeyBuffer.toString();
                    mKeyBuffer.setLength(0);
                    byte[] hash = getHash(source);
                    if (hash == null) continue;
                    BigInteger bi = new BigInteger(1, hash);  //制作正整数
                    int number = bi.testBit(0) ? 1 : 0; //生成0或者1，也可生成其它值
                    //int randomInt = bi.remainder(BigInteger.valueOf(2147483648L)).intValue(); //生成随机int值
                    if (number == 1) {
                        flagCount++;
                    }
                    eventCount++;
                    //numbers.add(number);
                    Dbg.print(number, eventCount, flagCount, (float) flagCount / eventCount);
                }
                Dbg.print("====end====");
            }
        }.start();
    }


    @Override
    protected void onDestroy() {
        super.onDestroy();

        sm.unregisterListener(this);

        isRunning = false;
    }

    private String makeEventKey(SensorEvent event) {
        StringBuilder sb = new StringBuilder();
        sb.append(event.timestamp);
        if (event.values != null) {
            for (float value : event.values) {
                sb.append("_").append(value);
            }
        }
        return sb.toString();
    }

    @Override
    public void onSensorChanged(SensorEvent event) {
        String key = makeEventKey(event);
        mKeyBuffer.append(key).append("_");
    }

    @Override
    public void onAccuracyChanged(Sensor sensor, int accuracy) {


    }

    public static byte[] getHash(String source) {
        MessageDigest digest = null;
        try {
            digest = MessageDigest.getInstance("SHA256");
            digest.update(source.getBytes());
            return digest.digest();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }