Java 抑制异常示例
原文： https://howtodoinjava.com/java7/java-suppressed-exceptions/
顾名思义， 被抑制的异常是在代码中引发的异常，但是以某种方式被忽略了。 如果您记得try-catch-finally块的执行顺序以及它们如何返回任何值或异常，那么您会记得，如果try中引发了异常，则也将抑制finally块中引发的异常。
在 Java 7 之前的版本中，您通过记录日志了解了这些异常（如果已实现），但是一旦finally块结束，您就无法控制这些类型的异常。
好吧，借助 Java 7 中的新特性，您还可以控制这些受抑制的异常。
Table of contents
1\. What are suppressed exceptions?
2\. Suppressed exception example
3\. Demonstration in different scenarios
1. 什么是禁止的异常？
在 Java 7 中，遇到抑制异常的最常见用例是 try-with-resources 语句。 当我们在try块中遇到异常时，应用程序将尝试关闭资源。 如果它遇到关闭AutoCloseable资源时可能发生的多个异常，则会将其他异常作为抑制的异常附加到主要异常上。
为了支持抑制的异常，在 JDK 7 中向Throwable类（Exception和Error类的父级）添加了新的构造器和两个新方法。
Throwable.getSupressed(); // Returns Throwable[]
Throwable.addSupressed(aThrowable);
2. 抑制异常的例子
例如，在写入输出流时，可以从try块引发一个异常，当try-with-resources语句尝试关闭流时，可以从该异常中引发最多两个异常。
如果从try块引发一个异常，并且从try-with-resources语句引发一个或多个异常，则将从try-with-resources语句引发的那些异常抑制，并且由该块引发的异常由closeStream()方法抛出。
您可以通过从try块引发的异常中调用Throwable.getSuppressed()方法来检索这些受抑制的异常。
3. 在不同情况下的示例
例如，我正在编写一个自动关闭的资源（即DirtyResource.java），无论我们尝试访问它还是关闭它，它都会引发异常。 这样，当以不同方式访问时，我们将能够看到不同的行为。
public class DirtyResource implements AutoCloseable
{
/**
* Need to call this method if you want to access this resource
* @throws RuntimeException no matter how you call this method
* */
public void accessResource()
{
throw new RuntimeException("I wanted to access this resource. Bad luck. Its dirty resource !!!");
}
/**
* The overridden closure method from AutoCloseable interface
* @throws Exception which is thrown during closure of this dirty resource
* */
@Override
public void close() throws Exception
{
throw new NullPointerException("Remember me. I am your worst nightmare !! I am Null pointer exception !!");
}
}
3.1 抑制异常之前的特性
package com.howtodoinjava.demo.core;
import static java.lang.System.err;
public class SuppressedExceptionDemoWithTryFinallyPrevious
{
/**
* Executable member function demonstrating suppressed exceptions
* One exception is lost if not added in suppressed exceptions list
*/
public static void memberFunction() throws Exception
{
DirtyResource resource= new DirtyResource();
try
{
resource.accessResource();
}
finally
{
resource.close();
}
}
public static void main(String[] arguments) throws Exception
{
try
{
memberFunction();
}
catch(Exception ex)
{
err.println("Exception encountered: " + ex.toString());
final Throwable[] suppressedExceptions = ex.getSuppressed();
final int numSuppressed = suppressedExceptions.length;
if (numSuppressed > 0)
{
err.println("tThere are " + numSuppressed + " suppressed exceptions:");
for (final Throwable exception : suppressedExceptions)
{
err.println("tt" + exception.toString());
}
}
}
}
}
Output:
Exception encountered: java.lang.NullPointerException: Remember me. I am your worst nightmare !! I am Null pointer exception !!
如您所见，仅捕获了一个异常，而第二个RuntimeException被抑制。
3.2 在 Java 7 中抑制异常之后
package com.howtodoinjava.demo.core;
import static java.lang.System.err;
public class SuppressedExceptionDemoWithTryFinallyNew
{
/**
* Executable member function demonstrating suppressed exceptions
* Suppressed expression is added back in primary exception
*/
public static void memberFunction() throws Exception
{
Throwable th = null;
DirtyResource resource= new DirtyResource();
try
{
resource.accessResource();
}
catch(Exception e)
{
th = e;
}
finally
{
try
{
resource.close();
}
catch(Exception e)
{
if(th != null)
{
e.addSuppressed(th); //Add to primary exception
throw e;
}
}
}
}
/**
* Executable function demonstrating suppressed exceptions.
*/
public static void main(String[] arguments) throws Exception
{
try
{
memberFunction();
}
catch(Exception ex)
{
err.println("Exception encountered: " + ex.toString());
final Throwable[] suppressedExceptions = ex.getSuppressed();
final int numSuppressed = suppressedExceptions.length;
if (numSuppressed > 0)
{
err.println("tThere are " + numSuppressed + " suppressed exceptions:");
for (final Throwable exception : suppressedExceptions)
{
err.println("tt" + exception.toString());
}
}
}
}
}
Output:
Exception encountered: java.lang.NullPointerException: Remember me. I am your worst nightmare !! I am Null pointer exception !!
There are 1 suppressed exceptions:
java.lang.RuntimeException: I wanted to access this resource. Bad luck. Its dirty resource !!!
在这里，在catch块中，我们可以访问两个异常。 一个作为主要异常，第二个作为受抑制异常。
3.3 成员函数中的try-with-resource块并捕获异常
package com.howtodoinjava.demo.core;
import static java.lang.System.err;
public class SuppressedExceptionDemoWithTryCatch
{
public static void memberFunction() throws Exception
{
try (DirtyResource resource= new DirtyResource())
{
resource.accessResource();
}
}
/**
* Executable member function demonstrating suppressed exceptions using try-with-resources
*/
public static void main(String[] arguments) throws Exception
{
try
{
memberFunction();
}
catch(Exception ex)
{
err.println("Exception encountered: " + ex.toString());
final Throwable[] suppressedExceptions = ex.getSuppressed();
final int numSuppressed = suppressedExceptions.length;
if (numSuppressed > 0)
{
err.println("tThere are " + numSuppressed + " suppressed exceptions:");
for (final Throwable exception : suppressedExceptions)
{
err.println("tt" + exception.toString());
}
}
}
}
}
Output:
Exception encountered: java.lang.RuntimeException: I wanted to access this resource. Bad luck. Its dirty resource !!!
There are 1 suppressed exceptions:
java.lang.NullPointerException: Remember me. I am your worst nightmare !! I am Null pointer exception !!
太好了！ 在成员函数中使用try-with-resource时，我们能够看到这两种异常。
3.4 默认try-with-resource块
package com.howtodoinjava.demo.core;
public class SuppressedExceptionDemoWithTryWithResource
{
/**
* Demonstrating suppressed exceptions using try-with-resources
*/
public static void main(String[] arguments) throws Exception
{
try (DirtyResource resource= new DirtyResource())
{
resource.accessResource();
}
}
}
Output:
Exception in thread "main" java.lang.RuntimeException: I wanted to access this resource. Bad luck. Its dirty resource !!!
at DirtyResource.accessResource(DirtyResource.java:9)
at SuppressedExceptionDemoWithTryWithResource.main(SuppressedExceptionDemoWithTryWithResource.java:12)
Suppressed: java.lang.NullPointerException: Remember me. I am your worst nightmare !! I am Null pointer exception !!
at DirtyResource.close(DirtyResource.java:19)
at SuppressedExceptionDemoWithTryWithResource.main(SuppressedExceptionDemoWithTryWithResource.java:13)
好吧，非常高兴看到包含抑制异常的完整信息的输出。
学习愉快！