本文主要通过编程方式说明一下Java中StringBuilder,在多线程时并发使用,可能出现线程安全问题,下面看一下代码。

StringBuilder多线程使用线程安全问题代码

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;
public class NotThreadSafe {
private static final int CHARS_PER_THREAD = 1_000_000;
private static final int NUMBER_OF_THREADS = 4;
private StringBuilder builder;
@Before
public void setUp() {
builder = new StringBuilder();
}
@Test
public void testStringBuilder() throws ExecutionException, InterruptedException {
Runnable appender = () -> {
for (int i = 0; i < CHARS_PER_THREAD; i++) {
builder.append('A');
}
};
ExecutorService executorService = Executors.newFixedThreadPool(NUMBER_OF_THREADS);
List<Future<?>> futures = new ArrayList<>();
for (int i = 0; i < NUMBER_OF_THREADS; i++) {
futures.add(executorService.submit(appender));
}
for (Future<?> future : futures) {
future.get();
}
executorService.shutdown();
String builtString = builder.toString();
Assert.assertEquals(CHARS_PER_THREAD * NUMBER_OF_THREADS, builtString.length());
}
}

通过矛盾法证明StringBuilder不是线程安全的。运行时,它总是像下面这样抛出异常:

java.util.concurrent.ExecutionException: java.lang.ArrayIndexOutOfBoundsException: 73726
at java.util.concurrent.FutureTask.report(FutureTask.java:122)
at java.util.concurrent.FutureTask.get(FutureTask.java:192)
at NotThreadSafe.testStringBuilder(NotThreadSafe.java:37)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: java.lang.ArrayIndexOutOfBoundsException: 73726
at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:650)
at java.lang.StringBuilder.append(StringBuilder.java:202)
at NotThreadSafe.lambda$testStringBuilder$0(NotThreadSafe.java:28)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

所以在多线程中使用StringBuilder,要注意线程安全问题。

或者

下面是使用JUnit 5测试,它涵盖了StringBuilderStringBuffer:

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
);
}
}

测试结果

AbstractStringBuilderTest.testStringBuilder: 
10000 total, 165 error, 5988 failed, 3847 passed.
AbstractStringBuilderTest.testStringBuffer:
10000 total, 10000 passed.