Java生成随機字元串

工具類RandomString如下：

import java.security.SecureRandom;
import java.util.Locale;
import java.util.Objects;
import java.util.Random;

public class RandomString {
    /**
     * 26個大寫字母
     */
    public static final String upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    /**
     * 26個小寫字母
     */
    public static final String lower = upper.toLowerCase(Locale.ROOT);

    /**
     * 數字
     */
    public static final String digits = "0123456789";

    /**
     * alphanum為26個大寫字母+26個小寫字母+10個數字。産生的随機字元串從此挑選字元生成
     */
    public static final String alphanum = upper + lower + digits;

    private final Random random;

    private final char[] symbols;

    private final char[] buf;

    public RandomString(int length, Random random, String symbols) {
        if (length < 1) throw new IllegalArgumentException();
        if (symbols.length() < 2) throw new IllegalArgumentException();
        this.random = Objects.requireNonNull(random);
        this.symbols = symbols.toCharArray();
        this.buf = new char[length];
    }

    /**
     * 通過字母數字字元串生成器生成随機字元串。
     */
    public RandomString(int length, Random random) {
        this(length, random, alphanum);
    }

    /**
     * 使用強随機數生成器生成length個随機字元串
     */
    public RandomString(int length) {
        this(length, new SecureRandom());
    }

    /**
     * 預設生成長度21的字元串
     */
    public RandomString() {
        this(21);
    }

    /**
     * 生成随機字元串
     */
    public String getString() {
        for (int idx = 0; idx < buf.length; ++idx) {
            buf[idx] = symbols[random.nextInt(symbols.length)];
        }
        return new String(buf);
    }
}
           

測試：

public class Maintest {
    public static void main(String[] args) {
    	//生成長度是100的随機字元串
        RandomString gen = new RandomString(100, ThreadLocalRandom.current());
        System.out.println(gen.getString());
    }
           

輸出：