如何获取e.printStackTrace()的内容

public static void main(String[] args) {  
        try {  
            String aa = "";  
            System.out.println(aa.substring(3));  
  
        } catch (Exception e) {  
            e.printStackTrace();  
            StringWriter sw = new StringWriter();  
            e.printStackTrace(new PrintWriter(sw, true));  
            String str = sw.toString();  
            System.out.println("==========");  
  
            System.out.println(str);  
        }  
    }  

java.lang.StringIndexOutOfBoundsException: String index out of range: -3  
    at java.lang.String.substring(Unknown Source)  
    at java.lang.String.substring(Unknown Source)  
    at Getc.main(Getc.java:16)  
==========  
java.lang.StringIndexOutOfBoundsException: String index out of range: -3  
    at java.lang.String.substring(Unknown Source)  
    at java.lang.String.substring(Unknown Source)  
    at Getc.main(Getc.java:16)