String、StringBuilder、StringBuffer 拼接测试

String、StringBuilder、StringBuffer

测试拼接速度

@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

利用 javap 查看字节码文件，寻找其中的差异

• 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."<init>":()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."<init>":()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."<init>":()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

若不是循环，String 和 StringBuilder 拼接的速度是差不多的

• 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."<init>":()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."<init>":()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