我正在使用Apache Commons的库生成一个字母数字字符串：

import org.apache.commons.lang3.RandomStringUtils;

String keyLength = 20;
RandomStringUtils.randomAlphanumeric(keylength);

它又快又简单！

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();
}

使用美元应简单如下：

// "0123456789" + "ABCDE...Z"
String validCharacters = $('0', '9').join() + $('A', 'Z').join();

String randomString(int length) {
    return $(validCharacters).shuffle().slice(length).toString();
}

@Test
public void buildFiveRandomStrings() {
    for (int i : $(5)) {
        System.out.println(randomString(12));
    }
}

它输出的内容如下：

DKL1SBH9UJWC
JH7P0IT21EA5
5DTI72EO6SFU
HQUMJTEBNF7Y
1HCR6SKYWGT7

我正在使用Apache Commons的库生成一个字母数字字符串：

import org.apache.commons.lang3.RandomStringUtils;

String keyLength = 20;
RandomStringUtils.randomAlphanumeric(keylength);

它又快又简单！

我使用的是一个非常简单的Java8解决方案。只需根据您的需求进行定制。

...
import java.security.SecureRandom;
...

//Generate a random String of length between 10 to 20.
//Length is also randomly generated here.
SecureRandom random = new SecureRandom();

String sampleSet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_";

int stringLength = random.ints(1, 10, 21).mapToObj(x -> x).reduce((a, b) -> a).get();

String randomString = random.ints(stringLength, 0, sampleSet.length() - 1)
        .mapToObj(x -> sampleSet.charAt(x))
        .collect(Collector
            .of(StringBuilder::new, StringBuilder::append,
                StringBuilder::append, StringBuilder::toString));

我们可以使用它生成如下的字母数字随机字符串（返回的字符串将强制包含一些非数字字符以及一些数字字符）：

public String generateRandomString() {
            
    String sampleSet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_";
    String sampleSetNumeric = "0123456789";
    
    String randomString = getRandomString(sampleSet, 10, 21);
    String randomStringNumeric = getRandomString(sampleSetNumeric, 10, 21);
    
    randomString = randomString + randomStringNumeric;
    
    //Convert String to List<Character>
    List<Character> list = randomString.chars()
            .mapToObj(x -> (char)x)
            .collect(Collectors.toList());
    
    Collections.shuffle(list);
    
    //This is needed to force a non-numeric character as the first String
    //Skip this for() if you don't need this logic

    for(;;) {
        if(Character.isDigit(list.get(0))) Collections.shuffle(list);
        else break;
    }
    
    //Convert List<Character> to String
    randomString = list.stream()
            .map(String::valueOf)
            .collect(Collectors.joining());
    
    return randomString;
    
}

//Generate a random number between the lower bound (inclusive) and upper bound (exclusive)
private int getRandomLength(int min, int max) {
    SecureRandom random = new SecureRandom();
    return random.ints(1, min, max).mapToObj(x -> x).reduce((a, b) -> a).get();
}

//Generate a random String from the given sample string, having a random length between the lower bound (inclusive) and upper bound (exclusive)
private String getRandomString(String sampleSet, int min, int max) {
    SecureRandom random = new SecureRandom();
    return random.ints(getRandomLength(min, max), 0, sampleSet.length() - 1)
    .mapToObj(x -> sampleSet.charAt(x))
    .collect(Collector
        .of(StringBuilder::new, StringBuilder::append,
            StringBuilder::append, StringBuilder::toString));
}

public static String RandomAlphanum(int length)
{
    String charstring = "abcdefghijklmnopqrstuvwxyz0123456789";
    String randalphanum = "";
    double randroll;
    String randchar;
    for (double i = 0; i < length; i++)
    {
        randroll = Math.random();
        randchar = "";
        for (int j = 1; j <= 35; j++)
        {
            if (randroll <= (1.0 / 36.0 * j))
            {
                randchar = Character.toString(charstring.charAt(j - 1));
                break;
            }
        }
        randalphanum += randchar;
    }
    return randalphanum;
}

我使用Math.random（）使用了一个非常原始的算法。为了增加随机性，可以直接实现util.Date类。尽管如此，它还是有效的。