Problem
I am afraid the test you have written is incorrect.
The main requirement is to share the same StringBuilder instance between different threads. Whereas you are creating a StringBuilder object for each thread.
The problem is that a new Threadsafe() initialises a new StringBuilder():
class Threadsafe {
...
StringBuilder sb = new StringBuilder(str);
...
}
class MyThread1 implements Runnable {
Threadsafe sf = new Threadsafe();
...
}
class MyThread2 implements Runnable {
Threadsafe sf = new Threadsafe();
...
}
Explanation
To prove the StringBuilder class is not thread-safe, you need to write a test where n threads (n > 1) append some stuff to the same instance simultaneously.
Being aware of the size of all the stuff you are going to append, you will be able to compare this value with the result of builder.toString().length():
final long SIZE = 1000; // max stream size
final StringBuilder builder = Stream
.generate(() -> "a") // generate an infinite stream of "a"
.limit(SIZE) // make it finite
.parallel() // make it parallel
.reduce(new StringBuilder(), StringBuilder::append, (b1, b2) -> b1);
// put each element in the builder
Assert.assertEquals(SIZE, builder.toString().length());
Since it is actually not thread-safe, you may have trouble getting the result.
An ArrayIndexOutOfBoundsException may be thrown because of the char[] AbstractStringBuilder#value array and the allocation mechanism which was not designed for multithreading use.
Test
Here is my JUnit 5 test which covers both StringBuilder and StringBuffer:
public class AbstractStringBuilderTest {
@RepeatedTest(10000)
public void testStringBuilder() {
testAbstractStringBuilder(new StringBuilder(), StringBuilder::append);
}
@RepeatedTest(10000)
public void testStringBuffer() {
testAbstractStringBuilder(new StringBuffer(), StringBuffer::append);
}
private <T extends CharSequence> void testAbstractStringBuilder(T builder, BiFunction<T, ? super String, T> accumulator) {
final long SIZE = 1000;
final Supplier<String> GENERATOR = () -> "a";
final CharSequence sequence = Stream
.generate(GENERATOR)
.parallel()
.limit(SIZE)
.reduce(builder, accumulator, (b1, b2) -> b1);
Assertions.assertEquals(
SIZE * GENERATOR.get().length(), // expected
sequence.toString().length() // actual
);
}
}
Results
AbstractStringBuilderTest.testStringBuilder:
10000 total, 165 error, 5988 failed, 3847 passed.
AbstractStringBuilderTest.testStringBuffer:
10000 total, 10000 passed.
Threadsafe sf = new Threadsafe()(in both your thread classes) => That means, your two threads are operating on differentThreadsafeinstances and thus on differentStringBuilderinstances! – Jinnycurl https://docs.oracle.com/javase/8/docs/api/java/lang/StringBuilder.html | grep "not safe for use by multiple threads" && echo "Not thread safe". It's documented not to be thread safe. You might not be able to prove it's not thread safe because the implementation might have changed so that it is; you shouldn't rely upon that property though, because there is no guarantee that it will continue to be thread-safe. – Amateurish