首页手记 String拼接在场景下的分析及对比

String拼接在场景下的分析及对比

标签：

Java 源码

概要

所有人在学习Java的时候，都被老师强调，字符串相加要用StringBuffer或者StringBuild，表面理解确实会快些，但是经过编译优化后，还会快吗？今天借助Jad反编译工具，看一下优化后，字符串直接相加在不同场景会怎么样。

环境

JDK1.8

IDEA2018.2

jad 1.5.8g

场景

主要分为以下四个场景：

简单的直接相加
多次相加
循环相加
有判断的相加

简单相加

源码

public class StringTest {
  public static void main(String[] args) {
    // String 直接拼接
    String a = "1111";
    String b = "22222";
    String c = "3333";
    System.out.println(a + b + c);
  }
}

编译后

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   StringTest.java

import java.io.PrintStream;

public class StringTest
{

    public StringTest()
    {
    }

    public static void main(String args[])
    {
        String a = "1111";
        String b = "22222";
        String c = "3333";
        System.out.println((new StringBuilder()).append(a).append(b).append(c).toString());
    }
}

多次相加

源码

public class StringTest {
  public static void main(String[] args) {
    // String 直接拼接
    String a = "1111";
    String b = "22222";
    String c = "3333";
    System.out.println(a + b + c);
    a = a + b;
    System.out.println(a + c);
  }
}

编译后

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   StringTest.java

import java.io.PrintStream;

public class StringTest
{

    public StringTest()
    {
    }

    public static void main(String args[])
    {
        String a = "1111";
        String b = "22222";
        String c = "3333";
        System.out.println((new StringBuilder()).append(a).append(b).append(c).toString());
        a = (new StringBuilder()).append(a).append(b).toString();
        System.out.println((new StringBuilder()).append(a).append(c).toString());
    }
}

循环场景

源码

public class StringTest {

  public static void main(String[] args) {
    String a = "1111";
    String b = "22222";
    //循环字符串拼接
    for (int i = 0; i < 100; i++) {
      a = a + b;
    }
    System.out.println(a);
    //循环 buffer拼接
    StringBuffer forBuffer = new StringBuffer(a);
    for (int i = 0; i < 100; i++) {
      forBuffer.append(b);
    }
    System.out.println(forBuffer.toString());
  }
}

编译后

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   StringTest.java

import java.io.PrintStream;

public class StringTest
{

    public StringTest()
    {
    }

    public static void main(String args[])
    {
        String a = "1111";
        String b = "22222";
        for(int i = 0; i < 100; i++)
            a = (new StringBuilder()).append(a).append(b).toString();

        System.out.println(a);
        StringBuffer forBuffer = new StringBuffer(a);
        for(int i = 0; i < 100; i++)
            forBuffer.append(b);

        System.out.println(forBuffer.toString());
    }
}

有判断

源码

public class StringTest {

  public static void main(String[] args) {
    String a = "1111";
    String b = "22222";
    int i = 3;
    //字符串拼接
    if (i % 2 == 0) {
      a = a + b;
    }
    System.out.println(a);
    //buffer拼接
    StringBuffer forBuffer = new StringBuffer(a);
    if (i % 2 == 0) {
      forBuffer.append(b);
    }
    System.out.println(forBuffer.toString());
  }
}

编译后

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   StringTest.java

import java.io.PrintStream;

public class StringTest
{

    public StringTest()
    {
    }

    public static void main(String args[])
    {
        String a = "1111";
        String b = "22222";
        int i = 3;
        if(i % 2 == 0)
            a = (new StringBuilder()).append(a).append(b).toString();
        System.out.println(a);
        StringBuffer forBuffer = new StringBuffer(a);
        if(i % 2 == 0)
            forBuffer.append(b);
        System.out.println(forBuffer.toString());
    }
}

性能

各种场景下，看到了优化器的结果，那就试试10W字符循环拼接下的，直接相加，Buffer，Builder相加的性能表现。

public class StringTest {

  public static void main(String[] args) {
    String a = "1111";
    String b = "22222";
    //循环字符串拼接
    long startTime = System.currentTimeMillis();
    for (int i = 0; i < 100000; i++) {
      a = a + b;
    }
    System.out.println(System.currentTimeMillis() - startTime);
    //循环 buffer拼接
    StringBuffer forBuffer = new StringBuffer(a);
    startTime = System.currentTimeMillis();
    for (int i = 0; i < 100000; i++) {
      forBuffer.append(b);
    }
    System.out.println(System.currentTimeMillis() - startTime);
    //循环 buffer拼接
    StringBuilder forBuilder = new StringBuilder(a);
    startTime = System.currentTimeMillis();
    for (int i = 0; i < 100000; i++) {
      forBuilder.append(b);
    }
    System.out.println(System.currentTimeMillis() - startTime);
  }
}