深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)

时间:2022-03-03 15:58:26

        String、StringBuffer、StringBuilder都是JAVA中常用的字符串操作类,对于他们的区别大家也都能耳熟能详,但底层到底是怎样实现的呢?今天就再深入分析下这三种字符串操作的区别、各自的原理及使用场景。

       请尊重作者劳动成果,转载请标明原文链接:

       https://www.cnblogs.com/jpcflyer/p/9280501.html

一、String

       先来看一下JDK中String中的部分源码:

深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)
public final class String implements java.io.Serializable, Comparable<String>, CharSequence { private final char value[]; private int hash; // Default to 0

     public String() { this.value = new char[0]; } public String(String original) { this.value = original.value; this.hash = original.hash; } public String(char value[]) { this.value = Arrays.copyOf(value, value.length); } ... }
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        可以看到String类、以及value都是final类型的,这样就表明String是无法被继承的,value是无法被改写的。当通过String的构造函数初始化新的String对象时,也只是根据传入的引用对象的value和hashcode进行了赋值。看下面的例子:

深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)
public class StringTest { public static void main(String[] args) { String str1 = "abc"; String str2 = "abc"; String Str3 = new String("abc"); } }
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       执行javac StringTest.java后,通过javap -v StringTest.class看下生成的class文件:

深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)
Classfile /C:/Users/jiang/workspace/test/src/test/StringTest.class Last modified 2018-7-8; size 363 bytes MD5 checksum f7e4243b0247fb20c5a336d4ba0a580f Compiled from "StringTest.java"
public class test.StringTest minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref          #6.#15         // java/lang/Object."<init>":()V
   #2 = String             #16            // abc
   #3 = Class              #17            // java/lang/String
   #4 = Methodref          #3.#18         // java/lang/String."<init>":(Ljava/lang/String;)V
   #5 = Class              #19            // test/StringTest
   #6 = Class              #20            // java/lang/Object
   #7 = Utf8               <init> #8 = Utf8 ()V #9 = Utf8 Code #10 = Utf8 LineNumberTable #11 = Utf8 main #12 = Utf8               ([Ljava/lang/String;)V #13 = Utf8 SourceFile #14 = Utf8 StringTest.java #15 = NameAndType        #7:#8          // "<init>":()V
  #16 = Utf8 abc #17 = Utf8               java/lang/String #18 = NameAndType        #7:#21         // "<init>":(Ljava/lang/String;)V
  #19 = Utf8               test/StringTest #20 = Utf8               java/lang/Object #21 = Utf8               (Ljava/lang/String;)V { public test.StringTest(); descriptor: ()V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1
         0: aload_0 1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return LineNumberTable: line 3: 0

  public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=3, locals=4, args_size=1
         0: ldc           #2                  // String abc
         2: astore_1 3: ldc           #2                  // String abc
         5: astore_2 6: new           #3                  // class java/lang/String
         9: dup 10: ldc           #2                  // String abc
        12: invokespecial #4                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
        15: astore_3 16: return LineNumberTable: line 6: 0 line 7: 3 line 8: 6 line 9: 16 } SourceFile: "StringTest.java"
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        可以看到对于相同的字符串“abc”的引用都是相同的(对于常量池中的相同位置),这样能够节省内存空间,但是缺点就是对于频繁的字符串拼接操作,会造成内存空间的浪费。(需要注意的是这种字符串的拼接操作,从JDK8 开始,会自动被编译成StringBuilder,是不是很666^_^,但还是建议不通过JDK途径去自动转。)看下面的代码:

深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)
public class StringTest { public static void main(String[] args) { String str1 = "abc"; //String str2 = "abc"; //String str3 = new String("abc");
        String str4 = str1 + "d"; String str5 = str4 + "e"; } }
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       然后再通过javap看下class文件:

深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)
Classfile /C:/Users/jiang/workspace/test/src/test/StringTest.class Last modified 2018-7-8; size 493 bytes MD5 checksum c02bd18ed3ecbe46f9859bf5e272c663 Compiled from "StringTest.java"
public class test.StringTest minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref          #10.#19        // java/lang/Object."<init>":()V
   #2 = String             #20            // abc
   #3 = Class              #21            // java/lang/StringBuilder
   #4 = Methodref          #3.#19         // java/lang/StringBuilder."<init>":()V
   #5 = Methodref          #3.#22         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   #6 = String             #23            // d
   #7 = Methodref          #3.#24         // java/lang/StringBuilder.toString:()Ljava/lang/String;
   #8 = String             #25            // e
   #9 = Class              #26            // test/StringTest
  #10 = Class              #27            // java/lang/Object
  #11 = Utf8               <init> #12 = Utf8 ()V #13 = Utf8 Code #14 = Utf8 LineNumberTable #15 = Utf8 main #16 = Utf8               ([Ljava/lang/String;)V #17 = Utf8 SourceFile #18 = Utf8 StringTest.java #19 = NameAndType        #11:#12        // "<init>":()V
  #20 = Utf8 abc #21 = Utf8               java/lang/StringBuilder #22 = NameAndType        #28:#29        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #23 = Utf8 d #24 = NameAndType        #30:#31        // toString:()Ljava/lang/String;
  #25 = Utf8 e #26 = Utf8               test/StringTest #27 = Utf8               java/lang/Object #28 = Utf8 append #29 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder; #30 = Utf8 toString #31 = Utf8               ()Ljava/lang/String; { public test.StringTest(); descriptor: ()V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1
         0: aload_0 1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return LineNumberTable: line 3: 0

  public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=2, locals=4, args_size=1
         0: ldc           #2                  // String abc
         2: astore_1 3: new           #3                  // class java/lang/StringBuilder
         6: dup 7: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
        10: aload_1 11: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        14: ldc           #6                  // String d
        16: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        19: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        22: astore_2 23: new           #3                  // class java/lang/StringBuilder
        26: dup 27: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
        30: aload_2 31: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        34: ldc           #8                  // String e
        36: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        39: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        42: astore_3 43: return LineNumberTable: line 6: 0 line 9: 3 line 10: 23 line 11: 43 } SourceFile: "StringTest.java"
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二、StringBuilder

       也是先来看StringBuilder的源码:

深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)
public final class StringBuilder extends AbstractStringBuilder implements java.io.Serializable, CharSequence { public StringBuilder() { super(16); } public StringBuilder(String str) { super(str.length() + 16); append(str); } public StringBuilder append(String str) { super.append(str); return this; } ... } abstract class AbstractStringBuilder implements Appendable, CharSequence { char[] value; int count; AbstractStringBuilder(int capacity) { value = new char[capacity]; } public AbstractStringBuilder append(String str) { if (str == null) return appendNull(); int len = str.length(); ensureCapacityInternal(count + len); str.getChars(0, len, value, count); count += len; return this; } ... }
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        可以看到StringBuilder的value是个char数组,(当然从JDK9开始,value从char数组变成了byte数组)。每次append时都是通过调用native的System.arraycopy实现的(在getChars中调用的)。

三、StringBuffer

S       tringBuffer的源码如下:

深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)深入理解String, StringBuffer, StringBuilder的区别(基于JDK1.8)
 public final class StringBuffer extends AbstractStringBuilder implements java.io.Serializable, CharSequence { private transient char[] toStringCache; public StringBuffer() { super(16); } public StringBuffer(String str) { super(str.length() + 16); append(str); } public synchronized StringBuffer append(String str) { toStringCache = null; super.append(str); return this; } ... }
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        和StringBuilder一样,都是用了char数组保存value,append也是调用了AbstractStringBuilder的append方法。区别只是在于char数组加了transient关键字,以及方法上加了synchronized方法。

       综上所述,String、StringBuilder、StringBuffer的使用场景如下:

       当处理定长字符串时,建议用String;

       当处理变长字符串时,并且是单线程环境时,建议用StringBuilder;

       当处理变长字符串时,并且是多线程环境时,建议用StringBuffer。