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

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

一、String

       先来看一下JDK中String中的部分源码：
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); } ... } View Code

        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：
public class StringTest { public static void main(String[] args) { String str1
= "abc"; String str2 = "abc"; String Str3 = new String("abc"); } } Vew Code
       执行javac StringTest.java后，通过javap -v StringTest.class看下生成的class文件：
Classfile /C:/Users/jiang/workspace/test/src/test/StringTest.class Last
modified2018-7-8; 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: line6: 0 line 7: 3 line 8: 6 line 9: 16 } SourceFile:
"StringTest.java" View Code

        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：
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"; } } View Code
       然后再通过javap看下class文件：
Classfile /C:/Users/jiang/workspace/test/src/test/StringTest.class Last
modified2018-7-8; 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: line6: 0 line 9: 3 line 10: 23 line 11: 43 } SourceFile:
"StringTest.java" View Code
二、StringBuilder

       也是先来看StringBuilder的源码：
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; } ... }
View Code
        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：
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; } ...
} View Code

        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。

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