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。
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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, 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); } ...}
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.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_SUPERConstant pool: #1 = Methodref #6.#15 // java/lang/Object."":()V #2 = String #16 // abc #3 = Class #17 // java/lang/String #4 = Methodref #3.#18 // java/lang/String." ":(Ljava/lang/String;)V #5 = Class #19 // test/StringTest #6 = Class #20 // java/lang/Object #7 = Utf8 #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 // " ":()V #16 = Utf8 abc #17 = Utf8 java/lang/String #18 = NameAndType #7:#21 // " ":(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." ":()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." ":(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"
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.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_SUPERConstant pool: #1 = Methodref #10.#19 // java/lang/Object."":()V #2 = String #20 // abc #3 = Class #21 // java/lang/StringBuilder #4 = Methodref #3.#19 // java/lang/StringBuilder." ":()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 #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 // " ":()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." ":()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." ":()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." ":()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"
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; } ...} 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; } ...} 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。