java 字符串拼接

阳光穿透心脏的1/2处 2022-05-14 09:45 351阅读 0赞

**> 字符串拼接一般使用“+”，但是“+”不能满足大批量数据的处理，Java中有以下五种方法处理字符串拼接，各有优缺点，程序开发应选择合适的方法实现。**

1. 加号 “+”

2. String contact() 方法

3. StringUtils.join() 方法

4. StringBuffer append() 方法

5. StringBuilder append() 方法

**> 经过简单的程序测试，从执行100次到90万次的时间开销如下表：**

![21037-20160930143043922-459833518.gif][]

由此可以看出：

1. 方法1 加号 “+” 拼接 和 方法2 String contact() 方法 适用于小数据量的操作，代码简洁方便，加号“+” 更符合我们的编码和阅读习惯；

2. 方法3 StringUtils.join() 方法 适用于将ArrayList转换成字符串，就算90万条数据也只需68ms，可以省掉循环读取ArrayList的代码；

3. 方法4 StringBuffer append() 方法 和 方法5 StringBuilder append() 方法 其实他们的本质是一样的，都是继承自AbstractStringBuilder，效率最高，大批量的数据处理最好选择这两种方法。

4. 方法1 加号 “+” 拼接 和 方法2 String contact() 方法 的时间和空间成本都很高（分析在本文末尾），不能用来做批量数据的处理。

**> 源代码，供参考**

![复制代码][copycode.gif]

package cnblogs.twzheng.lab2;
 
 /**
 * @author Tan Wenzheng
 *
 */
 import java.util.ArrayList;
 import java.util.List;
 
 import org.apache.commons.lang3.StringUtils;
 
 public class TestString {
 
 private static final int max = 100;
 
 public void testPlus() {
 System.out.println(">>> testPlus() <<<");
 
 String str = "";
 
 long start = System.currentTimeMillis();
 
 for (int i = 0; i < max; i++) {
 str = str + "a";
 }
 
 long end = System.currentTimeMillis();
 
 long cost = end - start;
 
 System.out.println(" {str + \"a\"} cost=" + cost + " ms");
 }
 
 public void testConcat() {
 System.out.println(">>> testConcat() <<<");
 
 String str = "";
 
 long start = System.currentTimeMillis();
 
 for (int i = 0; i < max; i++) {
 str = str.concat("a");
 }
 
 long end = System.currentTimeMillis();
 
 long cost = end - start;
 
 System.out.println(" {str.concat(\"a\")} cost=" + cost + " ms");
 }
 
 public void testJoin() {
 System.out.println(">>> testJoin() <<<");
 
 long start = System.currentTimeMillis();
 
 List<String> list = new ArrayList<String>();
 
 for (int i = 0; i < max; i++) {
 list.add("a");
 }
 
 long end1 = System.currentTimeMillis();
 long cost1 = end1 - start;
 
 StringUtils.join(list, "");
 
 long end = System.currentTimeMillis();
 long cost = end - end1;
 
 System.out.println(" {list.add(\"a\")} cost1=" + cost1 + " ms");
 System.out.println(" {StringUtils.join(list, \"\")} cost=" + cost
 + " ms");
 }
 
 public void testStringBuffer() {
 System.out.println(">>> testStringBuffer() <<<");
 
 long start = System.currentTimeMillis();
 
 StringBuffer strBuffer = new StringBuffer();
 
 for (int i = 0; i < max; i++) {
 strBuffer.append("a");
 }
 strBuffer.toString();
 
 long end = System.currentTimeMillis();
 
 long cost = end - start;
 
 System.out.println(" {strBuffer.append(\"a\")} cost=" + cost + " ms");
 }
 
 public void testStringBuilder() {
 System.out.println(">>> testStringBuilder() <<<");
 
 long start = System.currentTimeMillis();
 
 StringBuilder strBuilder = new StringBuilder();
 
 for (int i = 0; i < max; i++) {
 strBuilder.append("a");
 }
 strBuilder.toString();
 
 long end = System.currentTimeMillis();
 
 long cost = end - start;
 
 System.out
 .println(" {strBuilder.append(\"a\")} cost=" + cost + " ms");
 }
 }

![复制代码][copycode.gif]

**> 测试结果：**

1. 执行100次， private static final int max = 100;

![复制代码][copycode.gif]

>>> testPlus() <<<
 {str + "a"} cost=0 ms
 >>> testConcat() <<<
 {str.concat("a")} cost=0 ms
 >>> testJoin() <<<
 {list.add("a")} cost1=0 ms
 {StringUtils.join(list, "")} cost=20 ms
 >>> testStringBuffer() <<<
 {strBuffer.append("a")} cost=0 ms
 >>> testStringBuilder() <<<
 {strBuilder.append("a")} cost=0 ms

![复制代码][copycode.gif]

2. 执行1000次， private static final int max = 1000;

![复制代码][copycode.gif]

>>> testPlus() <<<
 {str + "a"} cost=10 ms
 >>> testConcat() <<<
 {str.concat("a")} cost=0 ms
 >>> testJoin() <<<
 {list.add("a")} cost1=0 ms
 {StringUtils.join(list, "")} cost=20 ms
 >>> testStringBuffer() <<<
 {strBuffer.append("a")} cost=0 ms
 >>> testStringBuilder() <<<
 {strBuilder.append("a")} cost=0 ms

![复制代码][copycode.gif]

3. 执行1万次， private static final int max = 10000;

![复制代码][copycode.gif]

>>> testPlus() <<<
 {str + "a"} cost=150 ms
 >>> testConcat() <<<
 {str.concat("a")} cost=70 ms
 >>> testJoin() <<<
 {list.add("a")} cost1=0 ms
 {StringUtils.join(list, "")} cost=30 ms
 >>> testStringBuffer() <<<
 {strBuffer.append("a")} cost=0 ms
 >>> testStringBuilder() <<<
 {strBuilder.append("a")} cost=0 ms

![复制代码][copycode.gif]

4. 执行10万次， private static final int max = 100000;

![复制代码][copycode.gif]

>>> testPlus() <<<
 {str + "a"} cost=4198 ms
 >>> testConcat() <<<
 {str.concat("a")} cost=1862 ms
 >>> testJoin() <<<
 {list.add("a")} cost1=21 ms
 {StringUtils.join(list, "")} cost=49 ms
 >>> testStringBuffer() <<<
 {strBuffer.append("a")} cost=10 ms
 >>> testStringBuilder() <<<
 {strBuilder.append("a")} cost=10 ms

![复制代码][copycode.gif]

5. 执行20万次， private static final int max = 200000;

![复制代码][copycode.gif]

>>> testPlus() <<<
 {str + "a"} cost=17196 ms
 >>> testConcat() <<<
 {str.concat("a")} cost=7653 ms
 >>> testJoin() <<<
 {list.add("a")} cost1=20 ms
 {StringUtils.join(list, "")} cost=51 ms
 >>> testStringBuffer() <<<
 {strBuffer.append("a")} cost=20 ms
 >>> testStringBuilder() <<<
 {strBuilder.append("a")} cost=16 ms

![复制代码][copycode.gif]

6. 执行50万次， private static final int max = 500000;

![复制代码][copycode.gif]

>>> testPlus() <<<
 {str + "a"} cost=124693 ms
 >>> testConcat() <<<
 {str.concat("a")} cost=49439 ms
 >>> testJoin() <<<
 {list.add("a")} cost1=21 ms
 {StringUtils.join(list, "")} cost=50 ms
 >>> testStringBuffer() <<<
 {strBuffer.append("a")} cost=20 ms
 >>> testStringBuilder() <<<
 {strBuilder.append("a")} cost=10 ms

![复制代码][copycode.gif]

7. 执行90万次， private static final int max = 900000;

![复制代码][copycode.gif]

>>> testPlus() <<<
 {str + "a"} cost=456739 ms
 >>> testConcat() <<<
 {str.concat("a")} cost=186252 ms
 >>> testJoin() <<<
 {list.add("a")} cost1=20 ms
 {StringUtils.join(list, "")} cost=68 ms
 >>> testStringBuffer() <<<
 {strBuffer.append("a")} cost=30 ms
 >>> testStringBuilder() <<<
 {strBuilder.append("a")} cost=24 ms

![复制代码][copycode.gif]

**> 查看源代码，以及简单分析**

String contact 和 StringBuffer，StringBuilder 的源代码都可以在Java库里找到，有空可以研究研究。

1. 其实每次调用contact()方法就是一次数组的拷贝，虽然在内存中是处理都是原子性操作，速度非常快，但是，最后的return语句会创建一个新String对象，限制了concat方法的速度。

![复制代码][copycode.gif]

public String concat(String str) {
            int otherLen = str.length();
            if (otherLen == 0) {
                return this;
            }
            int len = value.length;
            char buf[] = Arrays.copyOf(value, len + otherLen);
            str.getChars(buf, len);
            return new String(buf, true);
        }

![复制代码][copycode.gif]

2. StringBuffer 和 StringBuilder 的append方法都继承自AbstractStringBuilder，整个逻辑都只做**字符数组**的加长，拷贝，到最后也不会创建新的String对象，所以速度很快，完成拼接处理后在程序中用strBuffer.toString()来得到最终的字符串。

![复制代码][copycode.gif]

/**
 * Appends the specified string to this character sequence.
 * 
 * The characters of the {@code String} argument are appended, in
 * order, increasing the length of this sequence by the length of the
 * argument. If {@code str} is {@code null}, then the four
 * characters {@code "null"} are appended.
 * 
 * Let n be the length of this character sequence just prior to
 * execution of the {@code append} method. Then the character at
 * index k in the new character sequence is equal to the character
 * at index k in the old character sequence, if k is less
 * than n; otherwise, it is equal to the character at index
 * k-n in the argument {@code str}.
 *
 * @param str a string.
 * @return a reference to this object.
 */
 public AbstractStringBuilder append(String str) {
 if (str == null) str = "null";
 int len = str.length();
 ensureCapacityInternal(count + len);
 str.getChars(0, len, value, count);
 count += len;
 return this;
 }

![复制代码][copycode.gif]

/**
 * This method has the same contract as ensureCapacity, but is
 * never synchronized.
 */
 private void ensureCapacityInternal(int minimumCapacity) {
 // overflow-conscious code
 if (minimumCapacity - value.length > 0)
 expandCapacity(minimumCapacity);
 }
 
 /**
 * This implements the expansion semantics of ensureCapacity with no
 * size check or synchronization.
 */
 void expandCapacity(int minimumCapacity) {
 int newCapacity = value.length * 2 + 2;
 if (newCapacity - minimumCapacity < 0)
 newCapacity = minimumCapacity;
 if (newCapacity < 0) {
 if (minimumCapacity < 0) // overflow
 throw new OutOfMemoryError();
 newCapacity = Integer.MAX_VALUE;
 }
 value = Arrays.copyOf(value, newCapacity);
 }

![复制代码][copycode.gif]

3. 字符串的加号“+” 方法， 虽然编译器对其做了优化，使用StringBuilder的append方法进行追加，但是每循环一次都会创建一个StringBuilder对象，且都会调用toString方法转换成字符串，所以开销很大。

注：执行一次字符串“+”,相当于 str = new StringBuilder(str).append("a").toString();

4. 本文开头的地方统计了时间开销，根据上述分析再想想空间的开销。常说拿空间换时间，反过来是不是拿时间换到了空间呢，但是在这里，其实时间是消耗在了重复的不必要的工作上（生成新的对象，toString方法），所以对大批量数据做处理时，加号“+” 和 contact 方法绝对不能用，时间和空间成本都很高。

[21037-20160930143043922-459833518.gif]: /images/20220514/5447c8c1b07a4eb3a925a6d673f97809.png
[copycode.gif]: /images/20220514/485c6cc5f26c48a09a1a1a7ac231f4c8.png