String、StringBuilder、StringBuffer 拼接测试

摘要：测试拼接速度测试的结果在循环中，拼接字符串的速度远低于和利用查看字节码文件，寻找其中的差异命令行执行结果可以看出，拼接的时候也是通过的方法进行拼接的产生差异的原因是，在每次循环中，拼接的时候都了一个是线程安全的，只比稍慢了一点若不是

@Test
public void testString() {
    long before = System.currentTimeMillis();
    String s = "";
    for (int i = 0; i < 100000; i++) {
        s += "a";
    }
    long after = System.currentTimeMillis();
    System.out.println("String cost: " + (after - before));
}

@Test
public void testStringBuilder() {
    long before = System.currentTimeMillis();
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < 100000; i++) {
        sb.append("a");
    }
    long after = System.currentTimeMillis();
    System.out.println("StringBuilder cost: " + (after - before));
}

@Test
public void testStringBuffer() {
    long before = System.currentTimeMillis();
    StringBuffer sb = new StringBuffer();
    for (int i = 0; i < 100000; i++) {
        sb.append("a");
    }
    long after = System.currentTimeMillis();
    System.out.println("StringBuffer cost: " + (after - before));
}

测试的结果：

    StringBuilder cost: 5
    StringBuffer cost: 9
    String cost: 14029

在循环中，String 拼接字符串的速度远低于 StringBuilder 和 StringBuffer

命令行执行

javap -c StringTest.class

结果可以看出，String 拼接的时候也是通过 StringBuilder 的 append 方法进行拼接的

产生差异的原因是，在每次循环中，String 拼接的时候都 new 了一个 StringBuilder

StringBuffer 是线程安全的，只比 StringBuilder 稍慢了一点

public void testString();
Code:
0: ldc           #2                  // String
2: astore_1
3: iconst_0
4: istore_2
5: iload_2
6: ldc           #11                 // int 100000
8: if_icmpge     37
11: new           #4                  // class java/lang/StringBuilder
14: dup
15: invokespecial #5                  // Method java/lang/StringBuilder."":()V
18: aload_1
19: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
22: ldc           #9                  // String a
24: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
27: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
30: astore_1
31: iinc          2, 1
34: goto          5
37: return

public void testStringBuilder();
Code:
0: new           #4                  // class java/lang/StringBuilder
3: dup
4: invokespecial #5                  // Method java/lang/StringBuilder."":()V
7: astore_1
8: iconst_0
9: istore_2
10: iload_2
11: ldc           #11                 // int 100000
13: if_icmpge     29
16: aload_1
17: ldc           #9                  // String a
19: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
22: pop
23: iinc          2, 1
26: goto          10
29: return

public void testStringBuffer();
Code:
0: new           #12                 // class java/lang/StringBuffer
3: dup
4: invokespecial #13                 // Method java/lang/StringBuffer."":()V
7: astore_1
8: iconst_0
9: istore_2
10: iload_2
11: ldc           #11                 // int 100000
13: if_icmpge     29
16: aload_1
17: ldc           #9                  // String a
19: invokevirtual #14                 // Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
22: pop
23: iinc          2, 1
26: goto          10
29: return

java 测试代码

@Test
public void string() {
    String s = "";
    String ss = "cc";
    s += "a" + "b" + ss;
}
    
@Test
public void stringBuilder() {
    StringBuilder sb = new StringBuilder();
    String ss = "cc";
    sb.append("a").append("b").append(ss);
}

字节码

public void string();
Code:
0: ldc           #2                  // String
2: astore_1
3: ldc           #3                  // String cc
5: astore_2
6: new           #4                  // class java/lang/StringBuilder
9: dup
10: invokespecial #5                  // Method java/lang/StringBuilder."":()V
13: aload_1
14: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
17: ldc           #7                  // String ab
19: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
22: aload_2
23: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
26: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
29: astore_1
30: return

public void stringBuilder();
Code:
0: new           #4                  // class java/lang/StringBuilder
3: dup
4: invokespecial #5                  // Method java/lang/StringBuilder."":()V
7: astore_1
8: ldc           #3                  // String cc
10: astore_2
11: aload_1
12: ldc           #9                  // String a
14: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
17: ldc           #10                 // String b
19: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
22: aload_2
23: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
26: pop
27: return

原因是同样只 new 了一个 StringBuilder，使用的也是它的 append 方法