笔记-Java(JDK 1.9) StringBuffer 与 StringBuilder 的区别
StringBuffer 与 StringBuilder 都是为了更高效的操纵字符串而生，原因在于 String 类是不可变量，对其进行任何修改都会产生新的对象，而 StringBuffer 和 StringBuilder 则能够有效避免这一问题。但既然区分了两个类，就说明它们之间存在差别。
查看源码，发现 StringBuffer 与 StringBuilder 都继承 AbstractStringBuilder。下面从源码层面来对比两者
| StringBuffer | StringBuilder |
|---|---|
| public StringBuffer(int capacity) | public StringBuilder(int capacity) |
| public StringBuffer(String str) | public StringBuilder(String str) |
| public StringBuffer(CharSequence seq) | public StringBuilder(CharSequence seq) |
| public synchronized int length() | |
| public synchronized int capacity() | |
| public synchronized void ensureCapacity(int minimumCapacity) | |
| public synchronized void trimToSize() | |
| public synchronized void setLength(int newLength) | |
| public synchronized char charAt(int index) | |
| public synchronized int codePointAt(int index) | |
| public synchronized int codePointBefore(int index) | |
| public synchronized int codePointCount(int beginIndex, int endIndex) | |
| public synchronized int offsetByCodePoints(int index, int codePointOffset) | |
| public synchronized void getChars(int srcBegin, int srcEnd, char[] dst,int dstBegin) | |
| public synchronized void setCharAt(int index, char ch) | |
| public synchronized StringBuffer append(Object obj) | public StringBuilder append(Object obj) |
| public synchronized StringBuffer append(String str) | public StringBuilder append(String str) |
| public synchronized StringBuffer append(StringBuffer sb) | public StringBuilder append(StringBuffer sb) |
| synchronized StringBuffer append(AbstractStringBuilder asb) | |
| public synchronized StringBuffer append(CharSequence s) | public StringBuilder append(CharSequence s) |
| public synchronized StringBuffer append(CharSequence s, int start, int end) | public StringBuilder append(CharSequence s, int start, int end) |
| public synchronized StringBuffer append(char[] str) | public StringBuilder append(char[] str) |
| public synchronized StringBuffer append(char[] str, int offset, int len) | public StringBuilder append(char[] str, int offset, int len) |
| public synchronized StringBuffer append(boolean b) | public StringBuilder append(boolean b) |
| public synchronized StringBuffer append(char c) | public StringBuilder append(char c) |
| public synchronized StringBuffer append(int i) | public StringBuilder append(int i) |
| public synchronized StringBuffer appendCodePoint(int codePoint) | public StringBuilder appendCodePoint(int codePoint) |
| public synchronized StringBuffer append(long lng) | public StringBuilder append(long lng) |
| public synchronized StringBuffer append(float f) | public StringBuilder append(float f) |
| public synchronized StringBuffer append(double d) | public StringBuilder append(double d) |
| public synchronized StringBuffer delete(int start, int end) | public StringBuilder delete(int start, int end) |
| public synchronized StringBuffer deleteCharAt(int index) | public StringBuilder deleteCharAt(int index) |
| public synchronized StringBuffer replace(int start, int end, String str) | public StringBuilder replace(int start, int end, String str) |
| public synchronized String substring(int start) | |
| public synchronized CharSequence subSequence(int start, int end) | |
| public synchronized String substring(int start, int end) | |
| public synchronized StringBuffer insert(int index, char[] str, int offset, int len) | public StringBuilder insert(int index, char[] str, int offset, int len) |
| public synchronized StringBuffer insert(int offset, Object obj) | public StringBuilder insert(int offset, Object obj) |
| public synchronized StringBuffer insert(int offset, String str) | public StringBuilder insert(int offset, String str) |
| public synchronized StringBuffer insert(int offset, char[] str) | public StringBuilder insert(int offset, char[] str) |
| public StringBuffer insert(int dstOffset, CharSequence s) | public StringBuilder insert(int dstOffset, CharSequence s) |
| public synchronized StringBuffer insert(int dstOffset, CharSequence s, int start, int end) | public StringBuilder insert(int dstOffset, CharSequence s, int start, int end) |
| public StringBuffer insert(int offset, boolean b) | public StringBuilder insert(int offset, boolean b) |
| public synchronized StringBuffer insert(int offset, char c) | public StringBuilder insert(int offset, char c) |
| public StringBuffer insert(int offset, int i) | public StringBuilder insert(int offset, int i) |
| public StringBuffer insert(int offset, long l) | public StringBuilder insert(int offset, long l) |
| public StringBuffer insert(int offset, float f) | public StringBuilder insert(int offset, float f) |
| public StringBuffer insert(int offset, double d) | public StringBuilder insert(int offset, double d) |
| public int indexOf(String str) | public int indexOf(String str) |
| public synchronized int indexOf(String str, int fromIndex) | public int indexOf(String str, int fromIndex) |
| public int lastIndexOf(String str) | public int lastIndexOf(String str) |
| public synchronized int lastIndexOf(String str, int fromIndex) | public int lastIndexOf(String str, int fromIndex) |
| public synchronized StringBuffer reverse() | public StringBuilder reverse() |
| public synchronized String toString() | public String toString() |
| private synchronized void writeObject(java.io.ObjectOutputStream s) | private void writeObject(java.io.ObjectOutputStream s) |
| private void readObject(java.io.ObjectInputStream s) | private void readObject(java.io.ObjectInputStream s) |
| synchronized void getBytes(byte dst[], int dstBegin, byte coder) |
可以看到，StringBuffer 在几乎每个方法前面都有 synchronized 关键字修饰，即这个类是线程安全的，而在实现上两者都大致相同，比如 append(String str) 方法
//StringBuffer
public synchronized StringBuffer append(String str) {
toStringCache = null;
super.append(str);
return this;
}
//StringBuilder
public StringBuilder append(String str) {
super.append(str);
return this;
}
由于线程同步需要额外的性能消耗，所以在单线程环境中更适合使用 StringBuilder。