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顾名思义，抑制异常是在代码中抛出但以某种方式被忽略的异常。如果您还记得try-catch-finally块的执行顺序以及它们如何返回任何值或异常，您会记得在try块中抛出异常的同时，如果在finally块中抛出异常，那么finally块中抛出的异常会被抑制。
在Java 7之前，如果实现了日志记录，您会通过日志记录来了解这些异常，但一旦finally块结束，您就无法控制这些类型的异常。
好吧，借助Java 7 中的新功能，您还可以控制这些被抑制的异常。
1. 什么是抑制异常？
在Java 7中，也许遇到抑制异常最常见的情况是在使用try-with-resources语句时。当我们在try块内遇到异常时，应用程序会尝试关闭资源。如果在关闭AutoCloseable资源时发生多个异常，额外的异常会作为被抑制的异常附加到主要异常上。 为了支持被抑制的异常，在JDK 7中向Throwable类（Exception和Error类的父类）添加了一个新的构造函数和两个新方法。
Throwable.getSupressed(); // Returns Throwable[]
Throwable.addSupressed(aThrowable);
2. 抑制异常示例
例如，在写入输出流时，可以从块中引发异常try，并且当尝试关闭流时，可以从 try-with-resources 语句中引发最多两个异常。
如果 try 块中抛出了一个异常，并且 try-with-resources 语句中抛出了一个或多个异常，则 try-with-resources 语句中抛出的这些异常将被抑制，并且该块抛出的异常是第一个该方法抛出的closeStream()。
您可以通过从 try 块引发的异常中调用Throwable.getSuppressed()方法来检索这些被抑制的异常。
3、不同场景演示
例如，我正在编写一个自动可关闭资源（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，当以不同的方式访问时，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. 抑制异常功能之前
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 !!
正如您所看到的，只捕获了一个异常，并且第二个 RuntimeException 被抑制。
3.2. Java 7 中抑制异常支持之后
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 块中我们可以访问这两个异常。第一个作为主要异常，第二个作为抑制异常。
3.3. 成员函数中的 try-with-resource 块并捕获异常
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 !!
相当棒！！在成员函数中使用 try-with-resource 时，我们可以看到这两个异常。
3.4. 默认 try-with-resource 块
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)
嗯，看到包含完整信息和抑制异常的输出真是太棒了。
