// OutputStreamAppender.java
try {
    // skip some irrelevant codes...
    writeOut(event)
} catch (IOException ioe) {
    this.started = false;
    addStatus(new ErrorStatus("IO failure in appender", this, ioe));
}

// and writeOut is overrided by FileAppender.java
@Override
protected void writeOut(E event) throws IOException {
    if (prudent) {
        safeWrite(event); // here is the point, it ensure write integrity with file lock
    } else {
        super.writeOut(event);
    }
}

// OutputStreamAppender.java
protected void subAppend(E event) {
    if (!isStarted()) {
        return;
    }
    try {
        // skip some irrelevant codes...
        byte[] byteArray = this.encoder.encode(event);
        writeBytes(byteArray); // there is no guarantee to safe write
    } catch (IOException ioe) {
        this.started = false;
        addStatus(new ErrorStatus("IO failure in appender", this, ioe));
    }
}