StringBuilder和StringBuffer的區別

這里從兩個角度(性能、執行緒安全)去分析與區別，

StringBuilder與StringBuffer類結構

StringBuilder類繼承結構

StringBuffer類繼承結構

StringBuilder方法串列

StringBuffer方法串列

StringBuilder與StringBuffer性能比較

下面通過一個DEMO來對比StringBuilder與StringBuffer的性能，順帶測驗String的性能，

/**
 * Copyright (C), 2000-2021, XXX有限公司
 * FileName: StringBufferBuilderTest
 * Author: wangyetao
 * Date: 21-11-6 00:24:15
 * Description: StringBuilder和StringBuffer區別
 * <p>
 * History:
 * <author> 作者姓名
 * <time> 修改時間
 * <version> 版本號
 * <desc> 版本描述
 */
package simple.callback.stringbufferbuilder;

/**
 * @ClassName: StringBufferBuilderTest
 * @Description: java類描述
 * @Author: wangyetao
 * @Date: 21-11-6 00:24:15
 */
public class StringBufferBuilderTest {

    public static void main(String[] args) {
        //int performanceTestLength = 10000000;//1000萬
        int performanceTestLength = 10000;
        System.out.println("String性能測驗-length-" + performanceTestLength + ":" + appendStringPerformanceTest(performanceTestLength) + "ms");
        System.out.println("String2性能測驗-length-" + performanceTestLength + ":" + appendString2PerformanceTest(performanceTestLength) + "ms");
        System.out.println("StringBuffer性能測驗-length-" + performanceTestLength + ":" + appendStringBufferPerformanceTest(performanceTestLength) + "ms");
        System.out.println("StringBuilder性能測驗-length-" + performanceTestLength + ":" + appendStringBuilderPerformanceTest(performanceTestLength) + "ms");
    }

    /**
     * String性能測驗
     *
     * @param length 執行次數
     * @return 執行時長(ms)
     */
    private static long appendStringPerformanceTest(int length) {
        long start = System.currentTimeMillis();
        String str = "";
        for (int i = 0; i < length; i++) {
            str += "StringBuilder和StringBuffer區別";
        }
        return System.currentTimeMillis() - start;
    }

    /**
     * String性能測驗2
     *
     * @param length 執行次數
     * @return 執行時長(ms)
     */
    private static long appendString2PerformanceTest(int length) {
        long start = System.currentTimeMillis();
        String str = new String("");
        for (int i = 0; i < length; i++) {
            str += "StringBuilder和StringBuffer區別";
        }
        return System.currentTimeMillis() - start;
    }

    /**
     * StringBuffer性能測驗
     *
     * @param length 執行次數
     * @return 執行時長(ms)
     */
    private static long appendStringBufferPerformanceTest(int length) {
        long start = System.currentTimeMillis();
        StringBuffer stringBuffer = new StringBuffer();
        for (int i = 0; i < length; i++) {
            stringBuffer.append("StringBuilder和StringBuffer區別");
        }
        return System.currentTimeMillis() - start;
    }

    /**
     * StringBuilder性能測驗
     *
     * @param length 執行次數
     * @return 執行時長(ms)
     */
    private static long appendStringBuilderPerformanceTest(int length) {
        long start = System.currentTimeMillis();
        StringBuilder stringBuilder = new StringBuilder();
        for (int i = 0; i < length; i++) {
            stringBuilder.append("StringBuilder和StringBuffer區別");
        }
        return System.currentTimeMillis() - start;
    }



}

性能比較輸出結果：

String性能測驗-length-10000:2324ms
String2性能測驗-length-10000:1497ms
StringBuffer性能測驗-length-10000:1ms
StringBuilder性能測驗-length-10000:0ms

Process finished with exit code 0

String性能測驗-length-10000:1967ms
String2性能測驗-length-10000:1298ms
StringBuffer性能測驗-length-10000:1ms
StringBuilder性能測驗-length-10000:0ms

Process finished with exit code 0

String性能測驗-length-5000:702ms
String2性能測驗-length-5000:400ms
StringBuffer性能測驗-length-5000:1ms
StringBuilder性能測驗-length-5000:0ms

Process finished with exit code 0

StringBuilder與StringBuffer執行緒安全分析

從jdk原始碼上可以看到：

java.lang.StringBuilder#append(java.lang.String)不是執行緒安全的；

@Override
public StringBuilder append(String str) {
super.append(str);
return this;
}

java.lang.StringBuffer#append(java.lang.String)是執行緒安全的；

@Override
public synchronized StringBuffer append(String str) {
toStringCache = null;
super.append(str);
return this;
}

java.lang.AbstractStringBuilder#append(java.lang.String)

public AbstractStringBuilder append(String str) {
if (str == null)
return appendNull();
int len = str.length();
ensureCapacityInternal(count + len);
str.getChars(0, len, value, count);
count += len;
return this;
}

總結

記錄與總結，2021年 11月 06日 星期六 20:22:42 CST，