问题
What is the main difference between StringBuffer and StringBuilder?
Is there any performance issues when deciding on any one of these?
回答1:
StringBuffer is synchronized, StringBuilder is not.
回答2:
StringBuilder is faster than StringBuffer because it's not synchronized.
Here's a simple benchmark test:
public class Main {
public static void main(String[] args) {
int N = 77777777;
long t;
{
StringBuffer sb = new StringBuffer();
t = System.currentTimeMillis();
for (int i = N; i --> 0 ;) {
sb.append("");
}
System.out.println(System.currentTimeMillis() - t);
}
{
StringBuilder sb = new StringBuilder();
t = System.currentTimeMillis();
for (int i = N; i > 0 ; i--) {
sb.append("");
}
System.out.println(System.currentTimeMillis() - t);
}
}
}
A test run gives the numbers of 2241 ms for StringBuffer vs 753 ms for StringBuilder.
回答3:
Basically, StringBuffer methods are synchronized while StringBuilder are not.
The operations are "almost" the same, but using synchronized methods in a single thread is overkill.
That's pretty much about it.
Quote from StringBuilder API:
This class [StringBuilder] provides an API compatible with StringBuffer, but with no guarantee of synchronization. This class is designed for use as a drop-in replacement for StringBuffer in places where the string buffer was being used by a single thread (as is generally the case). Where possible, it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations.
So it was made to substitute it.
The same happened with Vector and ArrayList.
回答4:
But needed to get the clear difference with the help of an example?
StringBuffer or StringBuilder
Simply use StringBuilder unless you really are trying to share a buffer between threads. StringBuilder is the unsynchronized (less overhead = more efficient) younger brother of the original synchronized StringBuffer class.
StringBuffer came first. Sun was concerned with correctness under all conditions, so they made it synchronized to make it thread-safe just in case.
StringBuilder came later. Most of the uses of StringBuffer were single-thread and unnecessarily paying the cost of the synchronization.
Since StringBuilder is a drop-in replacement for StringBuffer without the synchronization, there would not be differences between any examples.
If you are trying to share between threads, you can use StringBuffer, but consider whether higher-level synchronization is necessary, e.g. perhaps instead of using StringBuffer, should you synchronize the methods that use the StringBuilder.
回答5:
First lets see the similarities: Both StringBuilder and StringBuffer are mutable. That means you can change the content of them, with in the same location.
Differences: StringBuffer is mutable and synchronized as well. Where as StringBuilder is mutable but not synchronized by default.
Meaning of synchronized (synchronization): When some thing is synchronized, then multiple threads can access, and modify it with out any problem or side effect. StringBuffer is synchronized, so you can use it with multiple threads with out any problem.
Which one to use when? StringBuilder : When you need a string, which can be modifiable, and only one thread is accessing and modifying it. StringBuffer : When you need a string, which can be modifiable, and multiple threads are accessing and modifying it.
Note : Don't use StringBuffer unnecessarily, i.e., don't use it if only one thread is modifying and accessing it because it has lot of locking and unlocking code for synchronization which will unnecessarily take up CPU time. Don't use locks unless it is required.
回答6:
In single threads, StringBuffer is not significantly slower than StringBuilder, thanks to JVM optimisations. And in multithreading, you can't use safely a StringBuilder.
Here is my test (not a benchmark, just a test) :
public static void main(String[] args) {
String withString ="";
long t0 = System.currentTimeMillis();
for (int i = 0 ; i < 100000; i++){
withString+="some string";
}
System.out.println("strings:" + (System.currentTimeMillis() - t0));
t0 = System.currentTimeMillis();
StringBuffer buf = new StringBuffer();
for (int i = 0 ; i < 100000; i++){
buf.append("some string");
}
System.out.println("Buffers : "+(System.currentTimeMillis() - t0));
t0 = System.currentTimeMillis();
StringBuilder building = new StringBuilder();
for (int i = 0 ; i < 100000; i++){
building.append("some string");
}
System.out.println("Builder : "+(System.currentTimeMillis() - t0));
}
Results :
strings: 319740
Buffers : 23
Builder : 7 !
So Builders are faster than Buffers, and WAY faster than strings concatenation. Now let's use an Executor for multiple threads :
public class StringsPerf {
public static void main(String[] args) {
ThreadPoolExecutor executorService = (ThreadPoolExecutor) Executors.newFixedThreadPool(10);
//With Buffer
StringBuffer buffer = new StringBuffer();
for (int i = 0 ; i < 10; i++){
executorService.execute(new AppendableRunnable(buffer));
}
shutdownAndAwaitTermination(executorService);
System.out.println(" Thread Buffer : "+ AppendableRunnable.time);
//With Builder
AppendableRunnable.time = 0;
executorService = (ThreadPoolExecutor) Executors.newFixedThreadPool(10);
StringBuilder builder = new StringBuilder();
for (int i = 0 ; i < 10; i++){
executorService.execute(new AppendableRunnable(builder));
}
shutdownAndAwaitTermination(executorService);
System.out.println(" Thread Builder: "+ AppendableRunnable.time);
}
static void shutdownAndAwaitTermination(ExecutorService pool) {
pool.shutdown(); // code reduced from Official Javadoc for Executors
try {
if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
pool.shutdownNow();
if (!pool.awaitTermination(60, TimeUnit.SECONDS))
System.err.println("Pool did not terminate");
}
} catch (Exception e) {}
}
}
class AppendableRunnable<T extends Appendable> implements Runnable {
static long time = 0;
T appendable;
public AppendableRunnable(T appendable){
this.appendable = appendable;
}
@Override
public void run(){
long t0 = System.currentTimeMillis();
for (int j = 0 ; j < 10000 ; j++){
try {
appendable.append("some string");
} catch (IOException e) {}
}
time+=(System.currentTimeMillis() - t0);
}
}
Now StringBuffers take 157 ms for 100000 appends. It's not the same test, but compared to the previous 37 ms, you can safely assume that StringBuffers appends are slower with multithreading use. The reason is that the JIT/hotspot/compiler/something makes optimizations when it detects that there is no need for checking locks.
But with StringBuilder, you have java.lang.ArrayIndexOutOfBoundsException, because a concurrent thread tries to add something where it should not.
Conclusion is that you don't have to chase StringBuffers. And where you have threads, think about what they are doing, before trying to gain a few nanoseconds.
回答7:
StringBuilder was introduced in Java 1.5 so it won't work with earlier JVMs.
From the Javadocs:
StringBuilder class provides an API compatible with StringBuffer, but with no guarantee of synchronization. This class is designed for use as a drop-in replacement for StringBuffer in places where the string buffer was being used by a single thread (as is generally the case). Where possible, it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations.
回答8:
Pretty Good Question
Here are the differences, i have noticed :
StringBuffer :-
StringBuffer is synchronized
StringBuffer is thread-safe
StringBuffer is slow (try to write a sample program and execute it, it will take more time than StringBuilder)
StringBuilder:-
StringBuilder is not synchronized
StringBuilder is not thread-safe
StringBuilder performance is better than StringBuffer.
Common thing :-
Both have same methods with same signatures. Both are mutable.
回答9:
StringBuilder is not thread safe. String Buffer is. More info here.
EDIT: As for performance , after hotspot kicks in , StringBuilder is the winner. However , for small iterations , the performance difference is negligible.
回答10:
StringBuilder and StringBuffer are almost the same. The difference is that StringBuffer is synchronized and StringBuilder is not. Although, StringBuilder is faster than StringBuffer, the difference in performance is very little. StringBuilder is a SUN's replacement of StringBuffer. It just avoids synchronization from all the public methods. Rather than that, their functionality is the same.
Example of good usage:
If your text is going to change and is used by multiple threads, then it is better to use StringBuffer. If your text is going to change but is used by a single thread, then use StringBuilder.
回答11:
StringBuffer
- Synchronized hence threadsafe
- Thread safe hence slow
StringBuilder
- Introduced in Java 5.0
- Asynchronous hence fast & efficient
- User explicitly needs to synchronize it, if he wants
- You can replace it with
StringBufferwithout any other change
回答12:
StringBuffer
StringBuffer is mutable means one can change the value of the object . The object created through StringBuffer is stored in the heap . StringBuffer has the same methods as the StringBuilder , but each method in StringBuffer is synchronized that is StringBuffer is thread safe .
because of this it does not allow two threads to simultaneously access the same method . Each method can be accessed by one thread at a time .
But being thread safe has disadvantages too as the performance of the StringBuffer hits due to thread safe property . Thus StringBuilder is faster than the StringBuffer when calling the same methods of each class.
StringBuffer value can be changed , it means it can be assigned to the new value . Nowadays its a most common interview question ,the differences between the above classes . String Buffer can be converted to the string by using toString() method.
StringBuffer demo1 = new StringBuffer(“Hello”) ;
// The above object stored in heap and its value can be changed .
demo1=new StringBuffer(“Bye”);
// Above statement is right as it modifies the value which is allowed in the StringBuffer
StringBuilder
StringBuilder is same as the StringBuffer , that is it stores the object in heap and it can also be modified . The main difference between the StringBuffer and StringBuilder is that StringBuilder is also not thread safe. StringBuilder is fast as it is not thread safe .
StringBuilder demo2= new StringBuilder(“Hello”);
// The above object too is stored in the heap and its value can be modified
demo2=new StringBuilder(“Bye”);
// Above statement is right as it modifies the value which is allowed in the StringBuilder
Resource: String Vs StringBuffer Vs StringBuilder
回答13:
String is an immutable.
StringBuffer is a mutable and synchronized.
StringBuilder is also mutable but its not synchronized.
回答14:
The javadoc explains the difference:
This class provides an API compatible with StringBuffer, but with no guarantee of synchronization. This class is designed for use as a drop-in replacement for StringBuffer in places where the string buffer was being used by a single thread (as is generally the case). Where possible, it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations.
回答15:
StringBuilder (introduced in Java 5) is identical to StringBuffer, except its methods are not synchronized. This means it has better performance than the latter, but the drawback is that it is not thread-safe.
Read tutorial for more details.
回答16:
A simple program illustrating the difference between StringBuffer and StringBuilder:
/**
* 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");
}
}
}
回答17:
Better use StringBuilder since it is not synchronized and therefor better performance. StringBuilder is a drop-in replacement of the older StringBuffer.
回答18:
StringBuffer is synchronized, but StringBuilder is not. As a result, StringBuilder is faster than StringBuffer.
回答19:
StringBuffer is mutable. It can change in terms of length and content. StringBuffers are thread-safe, meaning that they have synchronized methods to control access so that only one thread can access a StringBuffer object's synchronized code at a time. Thus, StringBuffer objects are generally safe to use in a multi-threaded environment where multiple threads may be trying to access the same StringBuffer object at the same time.
StringBuilder The StringBuilder class is very similar to StringBuffer, except that its access is not synchronized so that it is not thread-safe. By not being synchronized, the performance of StringBuilder can be better than StringBuffer. Thus, if you are working in a single-threaded environment, using StringBuilder instead of StringBuffer may result in increased performance. This is also true of other situations such as a StringBuilder local variable (ie, a variable within a method) where only one thread will be accessing a StringBuilder object.
回答20:
StringBuffer:
- Multi-Thread
- Synchronized
- Slow than StringBuilder
StringBuilder
- Single-Thread
- Not-Synchronized
- Faster than ever String
回答21:
String-Builder :
int one = 1;
String color = "red";
StringBuilder sb = new StringBuilder();
sb.append("One=").append(one).append(", Color=").append(color).append('\n');
System.out.print(sb);
// Prints "One=1, Colour=red" followed by an ASCII newline.
String-Buffer
StringBuffer sBuffer = new StringBuffer("test");
sBuffer.append(" String Buffer");
System.out.println(sBuffer);
It is recommended to use StringBuilder whenever possible because it is faster than StringBuffer. However, if the thread safety is necessary, the best option is StringBuffer objects.
回答22:
StringBuffer is used to store character strings that will be changed (String objects cannot be changed). It automatically expands as needed. Related classes: String, CharSequence.
StringBuilder was added in Java 5. It is identical in all respects to StringBuffer except that it is not synchronized, which means that if multiple threads are accessing it at the same time, there could be trouble. For single-threaded programs, the most common case, avoiding the overhead of synchronization makes the StringBuilder very slightly faster.
回答23:
There are no basic differences between StringBuilder and StringBuffer, only a few differences exist between them. In StringBuffer the methods are synchronized. This means that at a time only one thread can operate on them. If there are more than one thread then the second thread will have to wait for the first one to finish and the third one will have to wait for the first and second one to finish and so on. This makes the process very slow and hence the performance in case of StringBuffer is low.
On the other hand StringBuilder is non synchronized. This means that at a time multiple threads can operate on the same StrinBuilder object at the same time. This makes the process very fast and hence performance of StringBuilder is high.
回答24:
Since StringBuffer is synchronized, it needs some extra effort, hence based on perforamance, its a bit slow than StringBuilder.
回答25:
String is an immutable object which means the value cannot be changed where as StringBuffer is mutable.
The StringBuffer is Synchronized hence thread safe where as StringBuilder is not and suitable for only single threaded instances.
回答26:
The major difference is StringBuffer is syncronized but StringBuilder is not.If you need to use more than one thread , then StringBuffer is recommended.But, as per the execution speed StringBuilder is faster than StringBuffer , because its not syncronized .
回答27:
Check the internals of synchronized append method of StringBuffer and non-synchronized append method of StringBuilder.
StringBuffer:
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;
}
Since append is synchronized, StringBuffer has performance overhead compared to StrinbBuilder in multi-threading scenario. As long as you are not sharing buffer among multiple threads, use StringBuilder, which is fast due to absence of synchronized in append methods.
回答28:
Here is the performance testing result for String vs StringBuffer vs StringBuilder. Finally, StringBuilder won the Test. See below for test code and result.
Code:
private static void performanceTestStringVsStringbuffereVsStringBuilder() {
// String vs StringBiffer vs StringBuilder performance Test
int loop = 100000;
long start = 0;
// String
String str = null;
start = System.currentTimeMillis();
for (int i = 1; i <= loop; i++) {
str += i + "test";
}
System.out.println("String - " + (System.currentTimeMillis() - start) + " ms");
// String buffer
StringBuffer sbuffer = new StringBuffer();
start = System.currentTimeMillis();
for (int i = 1; i <= loop; i++) {
sbuffer.append(i).append("test");
}
System.out.println("String Buffer - " + (System.currentTimeMillis() - start) + " ms");
// String builder
start = System.currentTimeMillis();
StringBuilder sbuilder = new StringBuilder();
for (int i = 1; i <= loop; i++) {
sbuffer.append(i).append("test");
}
System.out.println("String Builder - " + (System.currentTimeMillis() - start) + " ms");
}
Execute Me on ideone
Result:
100000 iteration for adding a single text
String - 37489 ms
String Buffer - 5 ms
String Builder - 4 ms
10000 iteration for adding a single text
String - 389 ms
String Buffer - 1 ms
String Builder - 1 ms
回答29:
- StringBuffer is thread-safe but StringBuilder is not thread safe.
- StringBuilder is faster than StringBuffer.
- StringBuffer is synchronized whereas StringBuilder is not synchronized.
回答30:
StringBuffer is synchronized and thread safe, StringBuilder is not synchronised and faster.
来源：https://stackoverflow.com/questions/355089/difference-between-stringbuilder-and-stringbuffer