StringBuffer和StringBuilder之间的主要区别是什么?在决定其中任何一项时是否存在性能问题?
当前回答
StringBuilder和StringBuffer几乎相同。不同之处在于StringBuffer是同步的,而StringBuilder不是。尽管StringBuilder比StringBuffer更快,但性能差异很小。StringBuilder是SUN对StringBuffer的替代品。它只是避免了所有公共方法的同步。相反,它们的功能是相同的。
良好使用示例:
如果您的文本将要更改并被多个线程使用,那么最好使用StringBuffer。如果您的文本将要更改,但被单个线程使用,请使用StringBuilder。
其他回答
StringBuffer已同步,StringBuilder未同步。
一个简单的程序说明了StringBuffer和StringBuilder之间的区别:
/**
* Run this program a couple of times. We see that the StringBuilder does not
* give us reliable results because its methods are not thread-safe as compared
* to StringBuffer.
*
* For example, the single append in StringBuffer is thread-safe, i.e.
* only one thread can call append() at any time and would finish writing
* back to memory one at a time. In contrast, the append() in the StringBuilder
* class can be called concurrently by many threads, so the final size of the
* StringBuilder is sometimes less than expected.
*
*/
public class StringBufferVSStringBuilder {
public static void main(String[] args) throws InterruptedException {
int n = 10;
//*************************String Builder Test*******************************//
StringBuilder sb = new StringBuilder();
StringBuilderTest[] builderThreads = new StringBuilderTest[n];
for (int i = 0; i < n; i++) {
builderThreads[i] = new StringBuilderTest(sb);
}
for (int i = 0; i < n; i++) {
builderThreads[i].start();
}
for (int i = 0; i < n; i++) {
builderThreads[i].join();
}
System.out.println("StringBuilderTest: Expected result is 1000; got " + sb.length());
//*************************String Buffer Test*******************************//
StringBuffer sb2 = new StringBuffer();
StringBufferTest[] bufferThreads = new StringBufferTest[n];
for (int i = 0; i < n; i++) {
bufferThreads[i] = new StringBufferTest(sb2);
}
for (int i = 0; i < n; i++) {
bufferThreads[i].start();
}
for (int i = 0; i < n; i++) {
bufferThreads[i].join();
}
System.out.println("StringBufferTest: Expected result is 1000; got " + sb2.length());
}
}
// Every run would attempt to append 100 "A"s to the StringBuilder.
class StringBuilderTest extends Thread {
StringBuilder sb;
public StringBuilderTest (StringBuilder sb) {
this.sb = sb;
}
@Override
public void run() {
for (int i = 0; i < 100; i++) {
sb.append("A");
}
}
}
//Every run would attempt to append 100 "A"s to the StringBuffer.
class StringBufferTest extends Thread {
StringBuffer sb2;
public StringBufferTest (StringBuffer sb2) {
this.sb2 = sb2;
}
@Override
public void run() {
for (int i = 0; i < 100; i++) {
sb2.append("A");
}
}
}
StringBuffer已同步,但StringBuilder未同步。因此,StringBuilder比StringBuffer更快。
StringBuffer中的每个方法都是同步的。因此,一次只允许一个线程操作StringBuffer对象。它增加了线程的等待时间并造成性能问题为了解决这个问题,SUN People推出了1.5版本的StringBuilder。
检查StringBuffer的同步追加方法和StringBuilder的非同步追加方法的内部。
字符串缓冲区:
public StringBuffer(String str) {
super(str.length() + 16);
append(str);
}
public synchronized StringBuffer append(Object obj) {
super.append(String.valueOf(obj));
return this;
}
public synchronized StringBuffer append(String str) {
super.append(str);
return this;
}
StringBuilder:
public StringBuilder(String str) {
super(str.length() + 16);
append(str);
}
public StringBuilder append(Object obj) {
return append(String.valueOf(obj));
}
public StringBuilder append(String str) {
super.append(str);
return this;
}
由于追加是同步的,因此与多线程场景中的StrinbBuilder相比,StringBuffer具有性能开销。只要不在多个线程之间共享缓冲区,就可以使用StringBuilder,因为缺少同步的追加方法,所以速度很快。
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