String, StringBuffer, StringBuilder 举例探究效率

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在自己以往的学习及工作经验中，形成的概念：

String对象具有不变性，一旦String对象生成，就不可能在被改变；

StringBuffer 线程安全；

StringBuilder 线程不安全；

 

现针对以上三种Object，在自己机器上手写代码做测试，以验证三者的效率差异：

三种Object同时在3种不同的字符串拼接中，循环5w次，耗时对比：

 

Source Code: Test.java

public class Test {
	
	static int len = 50000;
	
	public static void main(String[] args) {
		System.out.println("---testOne()---");
		testOne();
		System.out.println("---testTwo()---");
		testTwo();
		System.out.println("---testThree()---");
		testThree();
	}

	
	static void testOne() {
		Long t1 = System.currentTimeMillis();
		for (int i = 0; i < len; i++) {
			String result1 = "String" + "and" + "String" + "append";
		}
		Long t2 = System.currentTimeMillis();

		for (int i = 0; i < len; i++) {
			StringBuffer sb = new StringBuffer();
			sb.append("String");
			sb.append("and");
			sb.append("String");
			sb.append("append");

		}
		Long t3 = System.currentTimeMillis();
		for (int i = 0; i < len; i++) {
			StringBuilder sb = new StringBuilder();
			sb.append("String");
			sb.append("and");
			sb.append("String");
			sb.append("append");
		}
		Long t4 = System.currentTimeMillis();
		System.out.println("String:" + (t2 - t1)+"ms");
		System.out.println("StringBuffer:" + (t3 - t2)+"ms");
		System.out.println("StringBuilder:" + (t4 - t3)+"ms");
	}

	static void testTwo() {
		String temp = "abcd";
		String temp2 = "abcd";
		String temp3 = "abcd";
		Long t1 = System.currentTimeMillis();
		for (int i = 0; i < len; i++) {
			String result1 = temp + temp2 + temp3;
		}
		Long t2 = System.currentTimeMillis();

		for (int i = 0; i < len; i++) {
			StringBuffer sb = new StringBuffer();
			sb.append(temp);
			sb.append(temp2);
			sb.append(temp3);

		}
		Long t3 = System.currentTimeMillis();
		for (int i = 0; i < len; i++) {
			StringBuilder sb = new StringBuilder();
			sb.append(temp);
			sb.append(temp2);
			sb.append(temp3);
		}
		Long t4 = System.currentTimeMillis();
		System.out.println("String:" + (t2 - t1)+"ms");
		System.out.println("StringBuffer:" + (t3 - t2)+"ms");
		System.out.println("StringBuilder:" + (t4 - t3)+"ms");
	}

	static void testThree() {
		String temp = "abcd";
		String temp2 = "abcd";
		String temp3 = "abcd";
		Long t1 = System.currentTimeMillis();
		for (int i = 0; i < len; i++) {
			String result1 = "String" + "and" + "String" + "append" + temp
					+ temp2 + temp3;
		}
		Long t2 = System.currentTimeMillis();

		for (int i = 0; i < len; i++) {
			StringBuffer sb = new StringBuffer();
			sb.append("String");
			sb.append("and");
			sb.append("String");
			sb.append("append");
			sb.append(temp);
			sb.append(temp2);
			sb.append(temp3);

		}
		Long t3 = System.currentTimeMillis();
		for (int i = 0; i < len; i++) {
			StringBuilder sb = new StringBuilder();
			sb.append("String");
			sb.append("and");
			sb.append("String");
			sb.append("append");
			sb.append(temp);
			sb.append(temp2);
			sb.append(temp3);
		}
		Long t4 = System.currentTimeMillis();
		System.out.println("String:" + (t2 - t1)+"ms");
		System.out.println("StringBuffer:" + (t3 - t2)+"ms");
		System.out.println("StringBuilder:" + (t4 - t3)+"ms");

	}
}

 

反编译Test.class

import java.io.PrintStream;

public class Test
{
  static int len = 50000;

  public static void main(String[] args) {
    System.out.println("---testOne()---");
    testOne();
    System.out.println("---testTwo()---");
    testTwo();
    System.out.println("---testThree()---");
    testThree();
  }

  static void testOne()
  {
    Long t1 = Long.valueOf(System.currentTimeMillis());
    for (int i = 0; i < len; i++) {
      String str = "StringandStringappend";
    }
    Long t2 = Long.valueOf(System.currentTimeMillis());

    for (int i = 0; i < len; i++) {
      StringBuffer sb = new StringBuffer();
      sb.append("String");
      sb.append("and");
      sb.append("String");
      sb.append("append");
    }

    Long t3 = Long.valueOf(System.currentTimeMillis());
    for (int i = 0; i < len; i++) {
      StringBuilder sb = new StringBuilder();
      sb.append("String");
      sb.append("and");
      sb.append("String");
      sb.append("append");
    }
    Long t4 = Long.valueOf(System.currentTimeMillis());
    System.out.println("String:" + (t2.longValue() - t1.longValue()) + "ms");
    System.out.println("StringBuffer:" + (t3.longValue() - t2.longValue()) + "ms");
    System.out.println("StringBuilder:" + (t4.longValue() - t3.longValue()) + "ms");
  }

  static void testTwo() {
    String temp = "abcd";
    String temp2 = "abcd";
    String temp3 = "abcd";
    Long t1 = Long.valueOf(System.currentTimeMillis());
    for (int i = 0; i < len; i++) {
      String str1 = temp + temp2 + temp3;
    }
    Long t2 = Long.valueOf(System.currentTimeMillis());

    for (int i = 0; i < len; i++) {
      StringBuffer sb = new StringBuffer();
      sb.append(temp);
      sb.append(temp2);
      sb.append(temp3);
    }

    Long t3 = Long.valueOf(System.currentTimeMillis());
    for (int i = 0; i < len; i++) {
      StringBuilder sb = new StringBuilder();
      sb.append(temp);
      sb.append(temp2);
      sb.append(temp3);
    }
    Long t4 = Long.valueOf(System.currentTimeMillis());
    System.out.println("String:" + (t2.longValue() - t1.longValue()) + "ms");
    System.out.println("StringBuffer:" + (t3.longValue() - t2.longValue()) + "ms");
    System.out.println("StringBuilder:" + (t4.longValue() - t3.longValue()) + "ms");
  }

  static void testThree() {
    String temp = "abcd";
    String temp2 = "abcd";
    String temp3 = "abcd";
    Long t1 = Long.valueOf(System.currentTimeMillis());
    for (int i = 0; i < len; i++) {
      String str1 = "StringandStringappend" + temp + 
        temp2 + temp3;
    }
    Long t2 = Long.valueOf(System.currentTimeMillis());

    for (int i = 0; i < len; i++) {
      StringBuffer sb = new StringBuffer();
      sb.append("String");
      sb.append("and");
      sb.append("String");
      sb.append("append");
      sb.append(temp);
      sb.append(temp2);
      sb.append(temp3);
    }

    Long t3 = Long.valueOf(System.currentTimeMillis());
    for (int i = 0; i < len; i++) {
      StringBuilder sb = new StringBuilder();
      sb.append("String");
      sb.append("and");
      sb.append("String");
      sb.append("append");
      sb.append(temp);
      sb.append(temp2);
      sb.append(temp3);
    }
    Long t4 = Long.valueOf(System.currentTimeMillis());
    System.out.println("String:" + (t2.longValue() - t1.longValue()) + "ms");
    System.out.println("StringBuffer:" + (t3.longValue() - t2.longValue()) + "ms");
    System.out.println("StringBuilder:" + (t4.longValue() - t3.longValue()) + "ms");
  }
}

 

测试运行结果：

 

将len设为50w，运行结果差异会更大；

 

总结：

对一个注重执行效率的系统，应该根据实际当中不同的业务需求，按需选用String, StringBuffer 或 StringBuilder。

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