static final String AB = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
static SecureRandom rnd = new SecureRandom();

String randomString(int len){
   StringBuilder sb = new StringBuilder(len);
   for(int i = 0; i < len; i++)
      sb.append(AB.charAt(rnd.nextInt(AB.length())));
   return sb.toString();
}

您提到“简单”，但如果其他人正在寻找符合更严格安全要求的产品，您可能需要看看jpwgen。jpwgen在Unix中以pwgen为模型，非常可配置。

public class Utils {
    private final Random RANDOM = new SecureRandom();
    private final String ALPHABET = "0123456789QWERTYUIOPASDFGHJKLZXCVBNMqwertyuiopasdfghjklzxcvbnm";

    private String generateRandomString(int length) {
        StringBuffer buffer = new StringBuffer(length);
        for (int i = 0; i < length; i++) {
            buffer.append(ALPHABET.charAt(RANDOM.nextInt(ALPHABET.length())));
        }
        return new String(buffer);
    } 
}

一个简单的解决方案，但它只使用小写和数字：

Random r = new java.util.Random ();
String s = Long.toString (r.nextLong () & Long.MAX_VALUE, 36);

大小约为12位数，以36为基数，这样就无法进一步改进。当然，您可以附加多个实例。

Java 8中的另一种选择是：

static final Random random = new Random(); // Or SecureRandom
static final int startChar = (int) '!';
static final int endChar = (int) '~';

static String randomString(final int maxLength) {
  final int length = random.nextInt(maxLength + 1);
  return random.ints(length, startChar, endChar + 1)
        .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
        .toString();
}

也许这有帮助

package password.generater;

import java.util.Random;

/**
 *
 * @author dell
 */
public class PasswordGenerater {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        int length= 11;
        System.out.println(generatePswd(length));

        // TODO code application logic here
    }
    static char[] generatePswd(int len){
        System.out.println("Your Password ");
        String charsCaps="ABCDEFGHIJKLMNOPQRSTUVWXYZ"; 
        String Chars="abcdefghijklmnopqrstuvwxyz";
        String nums="0123456789";
        String symbols="!@#$%^&*()_+-=.,/';:?><~*/-+";
        String passSymbols=charsCaps + Chars + nums +symbols;
        Random rnd=new Random();
        char[] password=new char[len];

        for(int i=0; i<len;i++){
            password[i]=passSymbols.charAt(rnd.nextInt(passSymbols.length()));
        }
      return password;

    }
}