背景

在频繁调用场景，如方法体内或者循环语句中，新定义Pattern会导致重复预编译正则表达式，降低程序执行效率。另外，在 JDK 中部分 入参为正则表达式格式的 API，如 String.replaceAll, String.split 等，也需要关注性能问题。

验证

正例：

将 Pattern 对象预编译，并在常量中声明。

    private static final String IP_V4 = "^(((\\d)|([1-9]\\d)|(1\\d{2})|(2[0-4]\\d)|(25[0-5]))\\.){3}((\\d)|([1-9]\\d)|(1\\d{2})|(2[0-4]\\d)|(25[0-5]))$";// Pattern 常量private static final Pattern IP_V4_PATTERN = Pattern.compile(IP_V4);public static boolean isValidIpv4V2(String input) {if (input == null) {return false;}return IP_V4_PATTERN.matcher(input).matches();}

反例：

每次调用时才声明 Pattern。

    private static final String IP_V4 = "^(((\\d)|([1-9]\\d)|(1\\d{2})|(2[0-4]\\d)|(25[0-5]))\\.){3}((\\d)|([1-9]\\d)|(1\\d{2})|(2[0-4]\\d)|(25[0-5]))$";public static boolean isValidIpv4V1(String input) {if (input == null) {return false;}Pattern pattern = Pattern.compile(IP_V4);return pattern.matcher(input).matches();}

测试代码：

package com.ysx.utils.pattern.performance;import org.junit.jupiter.api.Test;import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.TimeUnit;
import java.util.regex.Pattern;/*** @author youngbear* @email youngbear@aliyun.com* @date 2023-09-17 8:31* @blog <a href="https://blog.csdn.net/next_second">...</a>* @github <a href="https://github.com/YoungBear">...</a>* @description 正则表达式性能测试*/
public class PrecompilePerformanceTest {private static final String IP_V4 = "^(((\\d)|([1-9]\\d)|(1\\d{2})|(2[0-4]\\d)|(25[0-5]))\\.){3}((\\d)|([1-9]\\d)|(1\\d{2})|(2[0-4]\\d)|(25[0-5]))$";// Pattern 常量private static final Pattern IP_V4_PATTERN = Pattern.compile(IP_V4);// 缓存private static final Map<String, Pattern> cacheCompilePatternMap = new ConcurrentHashMap<>();public static boolean isValidIpv4V1(String input) {if (input == null) {return false;}Pattern pattern = Pattern.compile(IP_V4);return pattern.matcher(input).matches();}public static boolean isValidIpv4V2(String input) {if (input == null) {return false;}return IP_V4_PATTERN.matcher(input).matches();}public static boolean isValidIpv4V3(String input) {if (input == null) {return false;}if (!cacheCompilePatternMap.containsKey(IP_V4)) {cacheCompilePatternMap.put(IP_V4, Pattern.compile(IP_V4));}Pattern pattern = cacheCompilePatternMap.get(IP_V4);return pattern.matcher(input).matches();}@Testpublic void performanceTest() {String input1 = "192.168.12.13";long start = System.currentTimeMillis();for (int i = 0; i < 10000; i++) {isValidIpv4V1(input1);}long stop = System.currentTimeMillis();System.out.println("isValidIpv4V1, input1, consume: " + (stop - start) + "ms");String input2 = "192.168.12.13";long start2 = System.currentTimeMillis();for (int i = 0; i < 10000; i++) {isValidIpv4V1(input2);}long stop2 = System.currentTimeMillis();System.out.println("isValidIpv4V1, input2, consume: " + (stop2 - start2) + "ms");}@Testpublic void performanceV2Test() {String input1 = "192.168.12.13";long start = System.currentTimeMillis();for (int i = 0; i < 10000; i++) {isValidIpv4V2(input1);}long stop = System.currentTimeMillis();System.out.println("isValidIpv4V2, input1, consume: " + (stop - start) + "ms");String input2 = "192.168.12.13";long start2 = System.currentTimeMillis();for (int i = 0; i < 10000; i++) {isValidIpv4V2(input2);}long stop2 = System.currentTimeMillis();System.out.println("isValidIpv4V2, input2, consume: " + (stop2 - start2) + "ms");}@Testpublic void performanceV3Test() {String input1 = "192.168.12.13";long start = System.currentTimeMillis();for (int i = 0; i < 10000; i++) {isValidIpv4V3(input1);}long stop = System.currentTimeMillis();System.out.println("isValidIpv4V3, input1, consume: " + (stop - start) + "ms");String input2 = "192.168.12.13";long start2 = System.currentTimeMillis();for (int i = 0; i < 10000; i++) {isValidIpv4V3(input2);}long stop2 = System.currentTimeMillis();System.out.println("isValidIpv4V3, input2, consume: " + (stop2 - start2) + "ms");}}

执行结果：

isValidIpv4V1, input1, consume: 232ms
isValidIpv4V1, input2, consume: 74ms
isValidIpv4V2, input1, consume: 24ms
isValidIpv4V2, input2, consume: 19ms
isValidIpv4V3, input1, consume: 20ms
isValidIpv4V3, input2, consume: 12ms

根据执行结果，可以明显看到，预编译正则表达式可以提升性能。

总结

• 通常情况下，正则表达式为常量，所以可以将其作为常量量，在类编译时预编译。 private static final Pattern xxx_PATTERN = Pattern.compile("xxx");
• 对于动态的正则表达式，可以将其缓存，即缓存其 Pattern 结果。（参考 isValidIpv4V3 )。
• 另外，对于外部收入的正则表达式，一定要校验其安全性，防止 ReDos 攻击。