在Java中，当对象实例化失败时会发生什么？

发布于 2024-09-12 18:38:23 字数 238 浏览 3 评论 0原文

我来自 c++ 背景，我发现自己经常在 java 中这样做：

SomeClass sc=new SomeClass();

if(null!=sc)
{
    sc.doSomething();
}

我想知道的是，如果构造函数由于某种原因失败（比如可能没有足够的内存），变量 sc 中将会有什么。我可以' 找不到直接的答案，我担心我只是在浪费时间，因为也许如果新的操作员失败，程序就会崩溃吗？

原文

I come from a c++ background and I find myself constantly doing this in java:

SomeClass sc=new SomeClass();

if(null!=sc)
{
    sc.doSomething();
}

What I want to know is what will be in the variable sc if the constructor fails for some reason (like maybe not enough memory). I can'
t find a straight answer, and I am worried that I am just wasting my time because maybe if the new operator fails would the program just crash anyway?

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筱武穆 2024-09-19 18:38:23

Java 规范语言第三版彻底涵盖了您的问题：

12.5 创建新类实例
每当创建一个新的类实例时，都会为其分配内存空间，其中包含该类类型中声明的所有实例变量以及该类类型的每个超类中声明的所有实例变量，包括所有可能的实例变量。被隐藏。如果没有足够的空间可用于为对象分配内存，则类实例的创建会突然完成，并显示 OutOfMemoryError。否则，新对象中的所有实例变量（包括超类中声明的实例变量）都将初始化为其默认值。
在对新创建的对象的引用作为结果返回之前，将使用以下过程处理指定的构造函数以初始化新对象：[...]

因此， new 根本不可能表达式返回null。无论返回什么，如果执行正常完成，无论实例化的类如何，都将始终是有效的实例。

处理异常

一般来说，可能的异常通常使用 try-catch 块进行处理：

String someString = askFromUser();
try {
   int num = Integer.parseInt(someString);
   doSomethingWith(num);
} catch (NumberFormatException e) {
   complainAboutIt();
}

根据您的情况，您可以考虑将 new SomeClass() 放入 try< /code> 块带有相应的 catch (OutOfMemoryError e)，但这非常不典型。除非您打算在发生这种情况时做一些有意义的事情，否则在大多数典型情况下，最好不要捕获任何错误可能在程序执行过程中发生。

从文档中：

Error 是 Throwable 的子类，它指示合理的应用程序不应尝试捕获的严重问题>。大多数此类错误都是异常情况。
方法不需要在其 throws 子句中声明在方法执行期间可能抛出但未被捕获的 Error 的任何子类，因为这些错误是不应该发生的异常情况。

另请参阅

< a href="http://download-llnw.oracle.com/javase/tutorial/essential/exceptions/index.html" rel="noreferrer">Java 教程/异常

The Java Specification Language 3rd Edition covers your question thoroughly:

12.5 Creation of New Class Instances
Whenever a new class instance is created, memory space is allocated for it with room for all the instance variables declared in the class type and all the instance variables declared in each superclass of the class type, including all the instance variables that may be hidden. If there is not sufficient space available to allocate memory for the object, then creation of the class instance completes abruptly with an OutOfMemoryError. Otherwise, all the instance variables in the new object, including those declared in superclasses, are initialized to their default values.
Just before a reference to the newly created object is returned as the result, the indicated constructor is processed to initialize the new object using the following procedure: [...]

So it's simply not possible for a new expression to return null. Whatever is returned, if the execution completes normally, will always be a valid instanceof whatever class was instantiated.

Handling exceptions

Generally speaking, possible exceptions are usually handled with a try-catch block:

String someString = askFromUser();
try {
   int num = Integer.parseInt(someString);
   doSomethingWith(num);
} catch (NumberFormatException e) {
   complainAboutIt();
}

In your case, you may consider putting new SomeClass() in a try block with a corresponding catch (OutOfMemoryError e), but this is highly atypical. Unless you plan to do something meaningful when this happens, in most typical scenarios it's best to not catch any Error that may occur during your program execution.

From the documentation:

An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions.
A method is not required to declare in its throws clause any subclasses of Error that might be thrown during the execution of the method but not caught, since these errors are abnormal conditions that should never occur.