Try-catch是用来帮助异常处理的。这意味着它将在某种程度上帮助我们的系统变得更加健壮:尝试从意外事件中恢复。
我们怀疑在执行和指令(发送消息)时可能会发生一些事情,因此它被包含在try中。如果发生了几乎意想不到的事情,我们可以做些什么:我们编写捕获。我不认为我们调用只是为了记录异常。我认为catch块是为了给我们从错误中恢复的机会。
现在,假设我们从错误中恢复,因为我们可以修复错误。如果能再试一次,那就太好了:
try{ some_instruction(); }
catch (NearlyUnexpectedException e){
fix_the_problem();
retry;
}
这将很快陷入永恒循环,但让我们假设fix_the_problem返回true,然后重试。假设在Java中没有这样的东西,你将如何解决这个问题?你认为解决这个问题的最佳设计代码是什么?
这就像一个哲学问题,因为我已经知道Java不直接支持我所要求的内容。
我知道这里已经有很多类似的答案,我的答案也没有太大不同,但我还是会把它贴出来,因为它涉及一个具体的案例/问题。
当你在PHP中处理facebook Graph API时,你有时会得到一个错误,但立即重新尝试同样的事情会得到一个积极的结果(由于各种神奇的互联网原因,超出了这个问题的范围)。在这种情况下,不需要修复任何错误,而是简单地再试一次,因为有某种“facebook错误”。
这段代码在创建facebook会话后立即使用:
//try more than once because sometimes "facebook error"
$attempt = 3;
while($attempt-- > 0)
{
// To validate the session:
try
{
$facebook_session->validate();
$attempt = 0;
}
catch (Facebook\FacebookRequestException $ex)
{
// Session not valid, Graph API returned an exception with the reason.
if($attempt <= 0){ echo $ex->getMessage(); }
}
catch (\Exception $ex)
{
// Graph API returned info, but it may mismatch the current app or have expired.
if($attempt <= 0){ echo $ex->getMessage(); }
}
}
此外,通过让for循环计数为零($attempt——),可以很容易地改变将来的尝试次数。
下面是Java 8+的可重用和更通用的方法,不需要外部库:
public interface IUnreliable<T extends Exception>
{
void tryRun ( ) throws T;
}
public static <T extends Exception> void retry (int retryCount, IUnreliable<T> runnable) throws T {
for (int retries = 0;; retries++) {
try {
runnable.tryRun();
return;
} catch (Exception e) {
if (retries < retryCount) {
continue;
} else {
throw e;
}
}
}
}
用法:
@Test
public void demo() throws IOException {
retry(3, () -> {
new File("/tmp/test.txt").createNewFile();
});
}
这是一个老问题,但解决方案仍然重要。以下是我在Java 8中不使用任何第三方库的通用解决方案:
public interface RetryConsumer<T> {
T evaluate() throws Throwable;
}
public interface RetryPredicate<T> {
boolean shouldRetry(T t);
}
public class RetryOperation<T> {
private RetryConsumer<T> retryConsumer;
private int noOfRetry;
private int delayInterval;
private TimeUnit timeUnit;
private RetryPredicate<T> retryPredicate;
private List<Class<? extends Throwable>> exceptionList;
public static class OperationBuilder<T> {
private RetryConsumer<T> iRetryConsumer;
private int iNoOfRetry;
private int iDelayInterval;
private TimeUnit iTimeUnit;
private RetryPredicate<T> iRetryPredicate;
private Class<? extends Throwable>[] exceptionClasses;
private OperationBuilder() {
}
public OperationBuilder<T> retryConsumer(final RetryConsumer<T> retryConsumer) {
this.iRetryConsumer = retryConsumer;
return this;
}
public OperationBuilder<T> noOfRetry(final int noOfRetry) {
this.iNoOfRetry = noOfRetry;
return this;
}
public OperationBuilder<T> delayInterval(final int delayInterval, final TimeUnit timeUnit) {
this.iDelayInterval = delayInterval;
this.iTimeUnit = timeUnit;
return this;
}
public OperationBuilder<T> retryPredicate(final RetryPredicate<T> retryPredicate) {
this.iRetryPredicate = retryPredicate;
return this;
}
@SafeVarargs
public final OperationBuilder<T> retryOn(final Class<? extends Throwable>... exceptionClasses) {
this.exceptionClasses = exceptionClasses;
return this;
}
public RetryOperation<T> build() {
if (Objects.isNull(iRetryConsumer)) {
throw new RuntimeException("'#retryConsumer:RetryConsumer<T>' not set");
}
List<Class<? extends Throwable>> exceptionList = new ArrayList<>();
if (Objects.nonNull(exceptionClasses) && exceptionClasses.length > 0) {
exceptionList = Arrays.asList(exceptionClasses);
}
iNoOfRetry = iNoOfRetry == 0 ? 1 : 0;
iTimeUnit = Objects.isNull(iTimeUnit) ? TimeUnit.MILLISECONDS : iTimeUnit;
return new RetryOperation<>(iRetryConsumer, iNoOfRetry, iDelayInterval, iTimeUnit, iRetryPredicate, exceptionList);
}
}
public static <T> OperationBuilder<T> newBuilder() {
return new OperationBuilder<>();
}
private RetryOperation(RetryConsumer<T> retryConsumer, int noOfRetry, int delayInterval, TimeUnit timeUnit,
RetryPredicate<T> retryPredicate, List<Class<? extends Throwable>> exceptionList) {
this.retryConsumer = retryConsumer;
this.noOfRetry = noOfRetry;
this.delayInterval = delayInterval;
this.timeUnit = timeUnit;
this.retryPredicate = retryPredicate;
this.exceptionList = exceptionList;
}
public T retry() throws Throwable {
T result = null;
int retries = 0;
while (retries < noOfRetry) {
try {
result = retryConsumer.evaluate();
if (Objects.nonNull(retryPredicate)) {
boolean shouldItRetry = retryPredicate.shouldRetry(result);
if (shouldItRetry) {
retries = increaseRetryCountAndSleep(retries);
} else {
return result;
}
} else {
// no retry condition defined, no exception thrown. This is the desired result.
return result;
}
} catch (Throwable e) {
retries = handleException(retries, e);
}
}
return result;
}
private int handleException(int retries, Throwable e) throws Throwable {
if (exceptionList.contains(e.getClass()) || (exceptionList.isEmpty())) {
// exception is excepted, continue retry.
retries = increaseRetryCountAndSleep(retries);
if (retries == noOfRetry) {
// evaluation is throwing exception, no more retry left. Throw it.
throw e;
}
} else {
// unexpected exception, no retry required. Throw it.
throw e;
}
return retries;
}
private int increaseRetryCountAndSleep(int retries) {
retries++;
if (retries < noOfRetry && delayInterval > 0) {
try {
timeUnit.sleep(delayInterval);
} catch (InterruptedException ignore) {
Thread.currentThread().interrupt();
}
}
return retries;
}
}
让我们有一个这样的测试用例:
@Test
public void withPredicateAndException() {
AtomicInteger integer = new AtomicInteger();
try {
Integer result = RetryOperation.<Integer>newBuilder()
.retryConsumer(() -> {
int i = integer.incrementAndGet();
if (i % 2 == 1) {
throw new NumberFormatException("Very odd exception");
} else {
return i;
}
})
.noOfRetry(10)
.delayInterval(10, TimeUnit.MILLISECONDS)
.retryPredicate(value -> value <= 6)
.retryOn(NumberFormatException.class, EOFException.class)
.build()
.retry();
Assert.assertEquals(8, result.intValue());
} catch (Throwable throwable) {
Assert.fail();
}
}