java生成随机数案例(Java生成随机数的5种方式)(1)

1. Math.random() 静态方法

产生的随机数是 0 - 1 之间的一个 double ,即 0 <= random <= 1 。

使用:

for (int i = 0; i < 10; i ) { System.out.println(Math.random()); }

结果:

0.3598613895606426 0.2666778145365811 0.25090731064243355 0.011064998061666276 0.600686228175639 0.9084006027629496 0.12700524654847833 0.6084605849069343 0.7290804782514261 0.9923831908303121

实现原理:

When this method is first called, it creates a single new pseudorandom-number generator, exactly as if by the expression new java.util.Random() This new pseudorandom-number generator is used thereafter for all calls to this method and is used nowhere else.

当第一次调用 Math.random() 方法时,自动创建了一个伪随机数生成器 ,实际上用的是 new java.util.Random() 。当接下来继续调用 Math.random() 方法时,就会使用这个新的伪随机数生成器 。

源码如下:

public static double random() { Random rnd = randomNumberGenerator; if (rnd == null) rnd = initRNG(); // 第一次调用,创建一个伪随机数生成器 return rnd.nextDouble(); } private static synchronized Random initRNG() { Random rnd = randomNumberGenerator; return (rnd == null) ? (randomNumberGenerator = new Random()) : rnd; // 实际上用的是new java.util.Random() }

This method is properly synchronized to allow correct use by more than one thread. However, if many threads need to generate pseudorandom numbers at a great rate, it may reduce contention for each thread to have its own pseudorandom-number generator.

initRNG() 方法是 synchronized 的,因此在多线程情况下,只有一个线程会负责创建伪随机数生成器 (使用当前时间作为种子),其他线程则利用该伪随机数生成器 产生随机数。

因此 Math.random() 方法是线程安全的。什么情况下随机数的生成线程不安全:

什么情况下随机数的生成线程安全: Math.random() 静态方法使用

public class JavaRandom { public static void main(String args[]) { new MyThread().start(); new MyThread().start(); } } class MyThread extends Thread { public void run() { for (int i = 0; i < 2; i ) { System.out.println(Thread.currentThread().getName() ": " Math.random()); } } }

结果:

Thread-1: 0.8043581595645333 Thread-0: 0.9338269554390357 Thread-1: 0.5571569413128877 Thread-0: 0.37484586843392464

2. java.util.Random 工具类

基本算法:linear congruential pseudorandom number generator (LGC) 线性同余法伪随机数生成器 缺点:可预测

An attacker will simply compute the seed from the output values observed. This takes significantly less time than 2^48 in the case of java.util.Random. 从输出中可以很容易计算出种子值。It is shown that you can predict future Random outputs observing only two(!) output values in time roughly 2^16. 因此可以预测出下一个输出的随机数。 You should never use an LCG for security-critical purposes. 在注重信息安全的应用中,不要使用 LCG 算法生成随机数,请使用 SecureRandom。

使用:

Random random = new Random(); for (int i = 0; i < 5; i ) { System.out.println(random.nextInt()); }

结果:

-24520987 -96094681 -952622427 300260419 1489256498

Random类默认使用当前系统时钟作为种子:

public Random() { this(seedUniquifier() ^ System.nanoTime()); } public Random(long seed) { if (getClass() == Random.class) this.seed = new AtomicLong(initialScramble(seed)); else { // subclass might have overriden setSeed this.seed = new AtomicLong(); setSeed(seed); } }

Random类提供的方法:API

只要种子一样,产生的随机数也一样:因为种子确定,随机数算法也确定,因此输出是确定的!

Random random1 = new Random(10000); Random random2 = new Random(10000); for (int i = 0; i < 5; i ) { System.out.println(random1.nextInt() " = " random2.nextInt()); }

结果:

-498702880 = -498702880 -858606152 = -858606152 1942818232 = 1942818232 -1044940345 = -1044940345 1588429001 = 1588429001

3. java.util.concurrent.ThreadLocalRandom 工具类

ThreadLocalRandom 是 JDK 7 之后提供, 也是继承至 java.util.Random。

private static final ThreadLocal<ThreadLocalRandom> localRandom = new ThreadLocal<ThreadLocalRandom>() { protected ThreadLocalRandom initialValue() { return new ThreadLocalRandom(); } };

每一个线程有一个独立的 随机数生成器 ,用于并发产生随机数,能够解决多个线程发生的竞争争夺。 效率更高!

ThreadLocalRandom 不是直接用 new 实例化,而是第一次使用其静态方法 current() 得到 ThreadLocal<ThreadLocalRandom> 实例,然后调用 java.util.Random 类提供的方法获得各种随机数。

使用:

public class JavaRandom { public static void main(String args[]) { new MyThread().start(); new MyThread().start(); } } class MyThread extends Thread { public void run() { for (int i = 0; i < 2; i ) { System.out.println(Thread.currentThread().getName() ": " ThreadLocalRandom.current().nextDouble()); } } }

结果:

Thread-0: 0.13267085355389086 Thread-1: 0.1138484950410098 Thread-0: 0.17187774671469858 Thread-1: 0.9305225910262372

4. java.Security.SecureRandom也是继承至 java.util.Random。

Instances of java.util.Random are not cryptographically secure.Consider instead using SecureRandom to get a cryptographically secure pseudo-random number generator for use by security-sensitive applications. SecureRandom takes Random Data from your os (they can be interval between keystrokes etc - most os collect these data store them in files - /dev/random and /dev/urandom in case of linux/solaris) and uses that as the seed. 操作系统收集了一些随机事件,比如鼠标点击,键盘点击等等,SecureRandom 使用这些随机事件作为种子。

SecureRandom 提供加密的强随机数生成器 (RNG),要求种子必须是不可预知 的,产生非确定性 输出。 SecureRandom 也提供了与实现无关的算法,因此,调用方(应用程序代码)会请求特定的 RNG 算法并将它传回到该算法的 SecureRandom 对象中。

使用:

SecureRandom random1 = SecureRandom.getInstance("SHA1PRNG"); SecureRandom random2 = SecureRandom.getInstance("SHA1PRNG"); for (int i = 0; i < 5; i ) { System.out.println(random1.nextInt() " != " random2.nextInt()); }

结果:

704046703 != 2117229935 60819811 != 107252259 425075610 != -295395347 682299589 != -1637998900 -1147654329 != 1418666937

5. 随机字符串

可以使用 Apache Commons-Lang 包中的 RandomStringUtils 类。Maven 依赖如下:

<dependency> <groupId>commons-lang</groupId> <artifactId>commons-lang</artifactId> <version>2.6</version> </dependency>

API 参考:https://commons.apache.org/proper/commons-lang/javadocs/api-2.6/org/apache/commons/lang/RandomStringUtils.html

示例:

public class RandomStringDemo { public static void main(String[] args) { // Creates a 64 chars length random string of number. String result = RandomStringUtils.random(64, false, true); System.out.println("random = " result); // Creates a 64 chars length of random alphabetic string. result = RandomStringUtils.randomAlphabetic(64); System.out.println("random = " result); // Creates a 32 chars length of random ascii string. result = RandomStringUtils.randomAscii(32); System.out.println("random = " result); // Creates a 32 chars length of string from the defined array of // characters including numeric and alphabetic characters. result = RandomStringUtils.random(32, 0, 20, true, true, "qw32rfHIJk9iQ8Ud7h0X".toCharArray()); System.out.println("random = " result); } }

RandomStringUtils 类的实现上也是依赖了 java.util.Random 工具类:

java生成随机数案例(Java生成随机数的5种方式)(2)

RandomStringUtils 类的定义

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