本文主要是介绍深入理解String, StringBuffer, StringBuilder的区别（基于JDK1.8），希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
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。
一、String
先来看一下JDK中String中的部分源码：
public final class Stringimplements java.io.Serializable, Comparable<String>, CharSequence {private final char value[];private int hash; // Default to 0public 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);} ...
}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");}
}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.classLast modified 2018-7-8; size 363 bytesMD5 checksum f7e4243b0247fb20c5a336d4ba0a580fCompiled from "StringTest.java"
public class test.StringTestminor version: 0major version: 52flags: 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: ()Vflags: ACC_PUBLICCode:stack=1, locals=1, args_size=10: aload_01: invokespecial #1 // Method java/lang/Object."<init>":()V4: returnLineNumberTable:line 3: 0public static void main(java.lang.String[]);descriptor: ([Ljava/lang/String;)Vflags: ACC_PUBLIC, ACC_STATICCode:stack=3, locals=4, args_size=10: ldc #2 // String abc2: astore_13: ldc #2 // String abc5: astore_26: new #3 // class java/lang/String9: dup10: ldc #2 // String abc12: invokespecial #4 // Method java/lang/String."<init>":(Ljava/lang/String;)V15: astore_316: returnLineNumberTable:line 6: 0line 7: 3line 8: 6line 9: 16
}
SourceFile: "StringTest.java"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";}
}然后再通过javap看下class文件：
Classfile /C:/Users/jiang/workspace/test/src/test/StringTest.classLast modified 2018-7-8; size 493 bytesMD5 checksum c02bd18ed3ecbe46f9859bf5e272c663Compiled from "StringTest.java"
public class test.StringTestminor version: 0major version: 52flags: 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: ()Vflags: ACC_PUBLICCode:stack=1, locals=1, args_size=10: aload_01: invokespecial #1 // Method java/lang/Object."<init>":()V4: returnLineNumberTable:line 3: 0public static void main(java.lang.String[]);descriptor: ([Ljava/lang/String;)Vflags: ACC_PUBLIC, ACC_STATICCode:stack=2, locals=4, args_size=10: ldc #2 // String abc2: astore_13: new #3 // class java/lang/StringBuilder6: dup7: invokespecial #4 // Method java/lang/StringBuilder."<init>":()V10: aload_111: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;14: ldc #6 // String d16: 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_223: new #3 // class java/lang/StringBuilder26: dup27: invokespecial #4 // Method java/lang/StringBuilder."<init>":()V30: aload_231: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;34: ldc #8 // String e36: 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_343: returnLineNumberTable:line 6: 0line 9: 3line 10: 23line 11: 43
}
SourceFile: "StringTest.java"二、StringBuilder
也是先来看StringBuilder的源码：
public final class StringBuilderextends AbstractStringBuilderimplements 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;}...
}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
StringBuffer的源码如下：
public final class StringBufferextends AbstractStringBuilderimplements 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;}...
}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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