Java 输出 printStackTrace 堆栈信息

import java.io.PrintWriter;
import java.io.StringWriter;

public class MainDemo {
    private static String USER_NAME;

    public static void main(String[] args) {
        try {
            // 空指针异常
            System.out.println(USER_NAME.equals(""));
        } catch (Exception ex) {
            ex.printStackTrace();

            System.out.println("----------------------------");

            Exception exception = ex;
            StringWriter writer = new StringWriter();
            exception.printStackTrace(new PrintWriter(writer));
            System.out.println(writer.getBuffer().toString());
        }
    }
}

java.lang.NullPointerException
	at MainDemo.main(MainDemo.java:9)
----------------------------
java.lang.NullPointerException
	at MainDemo.main(MainDemo.java:9)