String和StringBuffer在字符串拼接上的性能辨析

    String和StringBuffer在字符串拼接性能上的比较是Java技术面试的常见问题,在很多书籍和文章中只是简单的说:“String是不可变类型,StringBuffer是可变类型;所以在字符串拼接时StringBuffer的效率要远高于String。”事实真的是这样吗?

 

1. 在只有字符串常量参与拼接的情况下,String的效率要远高于StringBuffer

    

package test;

public class StringTest {

	public static void main(String[] args) {
		long time1 = System.nanoTime();
		String hql = " select ui.real_name borrowerName,u.mobile mobile, u.email email,u.id borrowerId, " +
					 " l.loan_title loanTitle,l.open_amount openAmount,l.contact_amount contactAmount," +
					 " l.interest interest,l.loan_months loanMonths,p.name productName,l.loan_status loanStatus," +
					 " l.borrow_type borrowType, l.repayment_type repaymentType,l.id id,due.leaveAmount leaveAmount," +
					 " due.leaveNum leaveNum,c.channel_zn_name channelName" +
					 " from loan l left join user_info ui on l.borrower_id=ui.user_id" +
					 " left join user u on l.borrower_id=u.id" +
					 " left join product p on l.product_id=p.id" +
					 " left join channel c on c.channel_en_name=u.utm_source" +
					 " right join (select brr.loan_id loanId, sum(brr.repay_principal_interest) leaveAmount,count(id) leaveNum " +
					 " from borrow_repay_record brr where brr.status=:status or DATE_FORMAT(brr.repay_date,'%y-%m-%d') >= DATE_FORMAT(:leave_now,'%y-%m-%d')   " +
					 " GROUP BY brr.loan_id ) due on due.loanId=l.id " +
					 " where l.loan_status=:loan_status ";
		
		long time2 = System.nanoTime();
		
		StringBuffer sb = new StringBuffer();
		sb.append("select ui.real_name borrowerName,u.mobile mobile, u.email email,u.id borrowerId,")
		  .append("l.loan_title loanTitle,l.open_amount openAmount,l.contact_amount contactAmount," )
		  .append("l.interest interest,l.loan_months loanMonths,p.name productName,l.loan_status loanStatus," )
		  .append("l.borrow_type borrowType, l.repayment_type repaymentType,l.id id,due.leaveAmount leaveAmount," )
		  .append("due.leaveNum leaveNum,c.channel_zn_name channelName" )
		  .append("from loan l left join user_info ui on l.borrower_id=ui.user_id" )
		  .append("left join user u on l.borrower_id=u.id" )
		  .append("left join product p on l.product_id=p.id" )
		  .append("left join channel c on c.channel_en_name=u.utm_source" )
		  .append("right join (select brr.loan_id loanId, sum(brr.repay_principal_interest) leaveAmount,count(id) leaveNum")
		  .append("from borrow_repay_record brr where brr.status=:status or DATE_FORMAT(brr.repay_date,'%y-%m-%d') >= DATE_FORMAT(:leave_now,'%y-%m-%d')  ")
		  .append("GROUP BY brr.loan_id ) due on due.loanId=l.id")
		  .append("where l.loan_status=:loan_status");
		
		long time3 = System.nanoTime();
		
		System.out.println(time2-time1);
		System.out.println(time3-time2);
	}

}

 运行结果

 

8397
56915

 为什么会出现String的效率远高于StringBuffer的情况?这不是和书上说的相反了吗?

实际上这里是因为Java在编译java文件到class文件时,进行了优化,查看class文件:

// Compiled from StringTest.java (version 1.6 : 50.0, super bit)
public class test.StringTest {
  
  // Method descriptor #6 ()V
  // Stack: 1, Locals: 1
  public StringTest();
    0  aload_0 [this]
    1  invokespecial java.lang.Object() [8]
    4  return
      Line numbers:
        [pc: 0, line: 3]
      Local variable table:
        [pc: 0, pc: 5] local: this index: 0 type: test.StringTest
  
  // Method descriptor #15 ([Ljava/lang/String;)V
  // Stack: 5, Locals: 9
  public static void main(java.lang.String[] args);
      0  invokestatic java.lang.System.nanoTime() : long [16]
      3  lstore_1 [time1]
      4  ldc <String " select ui.real_name borrowerName,u.mobile mobile, u.email email,u.id borrowerId,  l.loan_title loanTitle,l.open_amount openAmount,l.contact_amount contactAmount, l.interest interest,l.loan_months loanMonths,p.name productName,l.loan_status loanStatus, l.borrow_type borrowType, l.repayment_type repaymentType,l.id id,due.leaveAmount leaveAmount, due.leaveNum leaveNum,c.channel_zn_name channelName from loan l left join user_info ui on l.borrower_id=ui.user_id left join user u on l.borrower_id=u.id left join product p on l.product_id=p.id left join channel c on c.channel_en_name=u.utm_source right join (select brr.loan_id loanId, sum(brr.repay_principal_interest) leaveAmount,count(id) leaveNum  from borrow_repay_record brr where brr.status=:status or DATE_FORMAT(brr.repay_date,'%y-%m-%d') >= DATE_FORMAT(:leave_now,'%y-%m-%d')    GROUP BY brr.loan_id ) due on due.loanId=l.id  where l.loan_status=:loan_status "> [22]
      6  astore_3 [hql]
      7  invokestatic java.lang.System.nanoTime() : long [16]
     10  lstore 4 [time2]
     12  new java.lang.StringBuffer [24]
     15  dup
     16  invokespecial java.lang.StringBuffer() [26]
     19  astore 6 [sb]
     21  aload 6 [sb]
     23  ldc <String "select ui.real_name borrowerName,u.mobile mobile, u.email email,u.id borrowerId,"> [27]
     25  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     28  ldc <String "l.loan_title loanTitle,l.open_amount openAmount,l.contact_amount contactAmount,"> [33]
     30  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     33  ldc <String "l.interest interest,l.loan_months loanMonths,p.name productName,l.loan_status loanStatus,"> [35]
     35  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     38  ldc <String "l.borrow_type borrowType, l.repayment_type repaymentType,l.id id,due.leaveAmount leaveAmount,"> [37]
     40  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     43  ldc <String "due.leaveNum leaveNum,c.channel_zn_name channelName"> [39]
     45  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     48  ldc <String "from loan l left join user_info ui on l.borrower_id=ui.user_id"> [41]
     50  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     53  ldc <String "left join user u on l.borrower_id=u.id"> [43]
     55  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     58  ldc <String "left join product p on l.product_id=p.id"> [45]
     60  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     63  ldc <String "left join channel c on c.channel_en_name=u.utm_source"> [47]
     65  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     68  ldc <String "right join (select brr.loan_id loanId, sum(brr.repay_principal_interest) leaveAmount,count(id) leaveNum"> [49]
     70  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     73  ldc <String "from borrow_repay_record brr where brr.status=:status or DATE_FORMAT(brr.repay_date,'%y-%m-%d') >= DATE_FORMAT(:leave_now,'%y-%m-%d')  "> [51]
     75  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     78  ldc <String "GROUP BY brr.loan_id ) due on due.loanId=l.id"> [53]
     80  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     83  ldc <String "where l.loan_status=:loan_status"> [55]
     85  invokevirtual java.lang.StringBuffer.append(java.lang.String) : java.lang.StringBuffer [29]
     88  pop
     89  invokestatic java.lang.System.nanoTime() : long [16]
     92  lstore 7 [time3]
     94  getstatic java.lang.System.out : java.io.PrintStream [57]
     97  lload 4 [time2]
     99  lload_1 [time1]
    100  lsub
    101  invokevirtual java.io.PrintStream.println(long) : void [61]
    104  getstatic java.lang.System.out : java.io.PrintStream [57]
    107  lload 7 [time3]
    109  lload 4 [time2]
    111  lsub
    112  invokevirtual java.io.PrintStream.println(long) : void [61]
    115  return
      Line numbers:
        [pc: 0, line: 6]
        [pc: 4, line: 7]
        [pc: 7, line: 21]
        [pc: 12, line: 23]
        [pc: 21, line: 24]
        [pc: 28, line: 25]
        [pc: 33, line: 26]
        [pc: 38, line: 27]
        [pc: 43, line: 28]
        [pc: 48, line: 29]
        [pc: 53, line: 30]
        [pc: 58, line: 31]
        [pc: 63, line: 32]
        [pc: 68, line: 33]
        [pc: 73, line: 34]
        [pc: 78, line: 35]
        [pc: 83, line: 36]
        [pc: 89, line: 38]
        [pc: 94, line: 40]
        [pc: 104, line: 41]
        [pc: 115, line: 42]
      Local variable table:
        [pc: 0, pc: 116] local: args index: 0 type: java.lang.String[]
        [pc: 4, pc: 116] local: time1 index: 1 type: long
        [pc: 7, pc: 116] local: hql index: 3 type: java.lang.String
        [pc: 12, pc: 116] local: time2 index: 4 type: long
        [pc: 21, pc: 116] local: sb index: 6 type: java.lang.StringBuffer
        [pc: 94, pc: 116] local: time3 index: 7 type: long
}

 在main方法的第四行很容易看出,编译时Java把多个字符串常量拼接成了一个长字符串,效率自然比使用StringBuffer高得多。

2. 有变量参与字符串拼接时,StringBuffer的效率要高于String

3. 对于有大量字符串常量参与拼接时可以将大段的字符串常量先用String的+进行拼接,再和其他部分使用StringBuffer进行拼接。

    

package test;

public class StringTest2 {

	public static void main(String[] args) {
		long time1 = System.nanoTime();
		String sql = " SELECT llr.id,llr.finance_plan_subpoint_id as financePlanSubPointId," +
			         " llr.loan_Id as loanId,llr.lend_amount as lendAmount,l.interest" + 
			         " FROM loan_lender_record llr JOIN loan l ON llr.loan_id = l.id " +
			         " WHERE llr.finance_plan_subpoint_id IN( SELECT id " +
			         "                                        FROM finance_plan_subpoint fps " +
			         "                                        WHERE fps.finance_plan_id = " + 
			         System.nanoTime() + 
			         ")";
		
		long time2 = System.nanoTime();
		
		StringBuffer sb = new StringBuffer();
		sb.append(" SELECT llr.id,llr.finance_plan_subpoint_id as financePlanSubPointId,")
		  .append(" llr.loan_Id as loanId,llr.lend_amount as lendAmount,l.interest" )
		  .append(" FROM loan_lender_record llr JOIN loan l ON llr.loan_id = l.id " )
		  .append(" WHERE llr.finance_plan_subpoint_id IN( SELECT id " )
		  .append("                                        FROM finance_plan_subpoint fps " )
		  .append("                                        WHERE fps.finance_plan_id = " )
		  .append(System.nanoTime())
		  .append(")");
		
		long time3 = System.nanoTime();

		String tempSql = " SELECT llr.id,llr.finance_plan_subpoint_id as financePlanSubPointId," +
			         " llr.loan_Id as loanId,llr.lend_amount as lendAmount,l.interest" + 
			         " FROM loan_lender_record llr JOIN loan l ON llr.loan_id = l.id " +
			         " WHERE llr.finance_plan_subpoint_id IN( SELECT id " +
			         "                                        FROM finance_plan_subpoint fps " +
			         "                                        WHERE fps.finance_plan_id = ";
 	    StringBuffer sb1 = new StringBuffer(tempSql); 
		sb1.append(System.nanoTime()).append(")");
		
		long time4 = System.nanoTime();
		
		System.out.println(time2-time1);
		System.out.println(time3-time2);
		System.out.println(time4-time3);
	}

}

 

    运行结果:

52249
41520
8397

 

4. 使用StringBuffer拼接字符串时,适当的直接初始化大小可以提升效率

    StringBuffer的底层是char数组,默认长度是16;当长度不足时按照“新长度=(原长度+1)*2” 来增大。

    // StringBuffer增大长度的实现
    void expandCapacity(int minimumCapacity) {
	int newCapacity = (value.length + 1) * 2;
        if (newCapacity < 0) {
            newCapacity = Integer.MAX_VALUE;
        } else if (minimumCapacity > newCapacity) {
	    newCapacity = minimumCapacity;
	}
        value = Arrays.copyOf(value, newCapacity);
    }

     如果字符串拼接的结果比较大,在使用StringBuffer时就会多次调用此函数,影响效率。所以可以直接在初始化时,设置好StringBuffer的长度。

 

 

字符串拼接的编码原则:

1. 全部由字符串常量组成的字符串进行拼接, 直接使用String的+操作即可。

2. 当有字符串变量参与拼接时,应当使用StringBuffer进行拼接操作。

3. 对于既有很多字符串常量也有字符串变量参与拼接,可以将大段的字符串常量先用String的+进行拼接,再和其他部分使用StringBuffer进行拼接。

4. 使用StringBuffer时,如果可以预估出结果长度,应该设置StringBuffer的初始化长度。

 

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