JVM - 【字符串常量池】-XX:StringTableSize

字符串常量池 - StringTable

---

一：配置常量池大小

-XX:StringTableSize=60013

• JDK1.6默认为1009，JDK1.7之后默认为60013，字符串常量池底层为HashTable，合理增大常量池大小会解决Hash冲突问题

• JDK1.8开始1009是可以设置的最小值

二：测试（常量池大小对执行效率的影响）

package xyz.xx.chapter_string;

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;

/**
 * -XX:StringTableSize=1009     ->  141ms
 * -XX:StringTableSize=100009   ->  43ms
 */
public class StringTest3 {
     
    public static void main(String[] args) {
     

        try {
     
            BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream("test.txt"), "UTF-8"));
            long start = System.currentTimeMillis();

            String data;
            while ((data = br.readLine()) != null) {
     
                data.intern();
            }

            long end = System.currentTimeMillis();
            System.out.println("耗時: "+(end-start)+"ms");
        }catch(IOException e){
     
            e.printStackTrace();
        }
    }
}

package xyz.xx.chapter_string;

import java.io.*;

/**
 * -XX:StringTableSize=1009
 * -XX:StringTableSize=100009
 */
public class StringTest2 {
     
    public static void main(String[] args) {
     
        try {
     
            BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(
                    new FileOutputStream("test.txt"),"UTF-8"));
            for(int i=0;i<100000;i++){
     
                int length = (int)(Math.random()*(10-1+1)+1);
                bw.write(getString(length) + "\n");
            }
            bw.close();
        } catch (IOException e) {
     
            e.printStackTrace();
        }
    }

    private static String getString(int length) {
     
        String res = "";
        for(int i=0;i<length;i++){
     
            // 65-90  97-122
            int num = (int)(Math.random()*(90-65+1)+65)+(int)(Math.random()*2)*32;
            res += (char)num;
        }
        return res;
    }
}

三：测试（字符串字面量的不变型）

package xyz.xx.chapter_string;

import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;

/**
 * hello
 * ABCDEFG
 * [?, B, C]
 * [hello, world]
 * 10
 */
public class StringTest1 {
     
    public static void main(String[] args) {
     
        String a = "ABCDEFG";
        char[] chs = {
     'A','B','C'};
        test1(a,chs);
        System.out.println(a);
        System.out.println(Arrays.toString(chs));

        List<String> list = new LinkedList<>();
        list.add("hello");
        list.add("world");
        test2(list);
        System.out.println(list);

        int number = 10;
        test3(number);
        System.out.println(number);
    }

    public static void test1(String str, char[] charArray){
     
        str = "hello";
        System.out.println(str);
        charArray[0] = '?';
    }

    public static void test2(List<String> list){
     
        list = new LinkedList<String>();
        list.add("test2");
    }

    public static void test3(int a){
     
        a = 99;
    }
}