Java字符串拼接的五種方法及性能比較分析(從執(zhí)行100次到90萬次)

更新時間：2021年12月17日 10:24:57 作者：老壇酸菜WH

字符串拼接一般使用“+”，但是“+”不能滿足大批量數(shù)據(jù)的處理，Java中有以下五種方法處理字符串拼接及性能比較分析，感興趣的可以了解一下

> 源代碼，供參考

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");
    }
}

> 測試結(jié)果：

1. 執(zhí)行100次， private static final int max = 100;

>>> 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

2. 執(zhí)行1000次， private static final int max = 1000;

>>> 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

3. 執(zhí)行1萬次， private static final int max = 10000;

>>> 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

4. 執(zhí)行10萬次， private static final int max = 100000;

>>> 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

5. 執(zhí)行20萬次， private static final int max = 200000;

>>> 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

6. 執(zhí)行50萬次， private static final int max = 500000;

>>> 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

7. 執(zhí)行90萬次， private static final int max = 900000;

>>> 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

> 查看源代碼，以及簡單分析

String contact 和 StringBuffer，StringBuilder 的源代碼都可以在Java庫里找到，有空可以研究研究。

1. 其實(shí)每次調(diào)用contact()方法就是一次數(shù)組的拷貝，雖然在內(nèi)存中是處理都是原子性操作，速度非?？?，但是，最后的return語句會創(chuàng)建一個新String對象，限制了concat方法的速度。

    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);
    }

2. StringBuffer 和 StringBuilder 的append方法都繼承自AbstractStringBuilder，整個邏輯都只做字符數(shù)組的加長，拷貝，到最后也不會創(chuàng)建新的String對象，所以速度很快，完成拼接處理后在程序中用strBuffer.toString()來得到最終的字符串。

    /**
     * Appends the specified string to this character sequence.
     * <p>
     * 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.
     * <p>
     * Let <i>n</i> be the length of this character sequence just prior to
     * execution of the {@code append} method. Then the character at
     * index <i>k</i> in the new character sequence is equal to the character
     * at index <i>k</i> in the old character sequence, if <i>k</i> is less
     * than <i>n</i>; otherwise, it is equal to the character at index
     * <i>k-n</i> 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;
    }

    /**
     * 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);
    }

3. 字符串的加號“+” 方法，雖然編譯器對其做了優(yōu)化，使用StringBuilder的append方法進(jìn)行追加，但是每循環(huán)一次都會創(chuàng)建一個StringBuilder對象，且都會調(diào)用toString方法轉(zhuǎn)換成字符串，所以開銷很大。

　　注：執(zhí)行一次字符串“+”,相當(dāng)于 str = new StringBuilder(str).append("a").toString();

4. 本文開頭的地方統(tǒng)計了時間開銷，根據(jù)上述分析再想想空間的開銷。常說拿空間換時間，反過來是不是拿時間換到了空間呢，但是在這里，其實(shí)時間是消耗在了重復(fù)的不必要的工作上（生成新的對象，toString方法），所以對大批量數(shù)據(jù)做處理時，加號“+” 和 contact 方法絕對不能用，時間和空間成本都很高。

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