public String doLayout(ILoggingEvent event) {
        if (!isStarted()) {
            return CoreConstants.EMPTY_STRING;
        }
        return writeLoopOnConverters(event);
    }
@Test
    public void prefixConverterWithProperty() {

        try {
            String propertyKey = "px1953";
            String propertyVal = "pxVal";

            System.setProperty(propertyKey, propertyVal);

            String pattern = "%prefix(%logger %property{" + propertyKey + "}) %message";
            pl.setPattern(pattern);
            pl.start();

            String val = pl.doLayout(makeLoggingEvent("hello", null));

            assertEquals("logger=" + logger.getName() + " " + propertyKey + "=" + propertyVal + " hello", val);

        } finally {
            System.clearProperty("px");
        }
    }