Why main method is marked as public?

问题:

I have a question that why main method is marked as public?

According to an answer on stackoverflow, It is declared as static

"The method is static because otherwise there would be ambiguity: which constructor should be called?"

But, can anyone can explain why it is declared public always?

回答1:

The initialization software that starts your program must be able to see main so that it can call it.

回答2:

Because the JLS, Section 12.1.4, says so:

The method main must be declared public, static, and void. It must specify a formal parameter (§8.4.1) whose declared type is array of String.

If it's not public, then it won't be found; you'll get

Error: Main method not found in class Main, please define the main method as:
   public static void main(String[] args)

回答3:

I believe, the rational behind enforcing main as public is more to do with the language specification – rather than whether something could be achieved or not.

Refer: http://docs.oracle.com/javase/specs/jls/se7/html/jls-12.html#jls-12.1.4

The method main must be declared public, static, and void. It must specify a formal parameter (§8.4.1) whose declared type is array of String. Therefore, either of the following declarations is acceptable:

Java uses JNI launch java application will never have any issue in calling a private main – but this is more like jail-brake (like another jail-brake, where reflection API let you access private method) and definitely not in spirit of java specification.

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If I remember till JDK 1.3 – it was not mandatory from developer’s perspective. i.e. even a private main was being accepted by JRE. Though, it was not inline with JLS 1.3.

I tried searching JLS 1.3 for a reference, but could not get a link. But I found that it was reported as a bug by developers across world:

Please refer: http://bugs.java.com/bugdatabase/view_bug.do?bug_id=4155575

So, a fix was made in subsequent version to enforce rule stated by JLS.

Now, the point is why JLS writer enforced this rule at first place – I really have no idea. The only thing I can think of is that – to keep it “obvious” and non-confusing for developers.

回答4:

Because that is what is known as the "entry point" and if it is private, your program will not be able to run.

回答5:

Public - main method is called by JVM to run the method which is outside the scope of project therefore the access specifier has to be public to permit call from anywhere outside the application.

From coderanch

回答6:

When the code is executed, a JVM will be created and that will act as a container for the code we are trying to execute. Declaring this method as public allows the JVM to start the code execution. If method is private, JVM wont be able to call it. There are ways to call a private method as well (eg by using Reflection), but that is not a standard way to do things.

回答7:

In Java a function or variable in class in unaccessible untill an Object is created from the class, but the 'main' function is supposed to run at startup(without Object initiation) by JVM. So the 'main' is declared public as well as static so that it can be accessed outside class & without even an Object is created.

回答8:

Yes the standards say so that main method should be public in Java. But it's not only for Java. However even for C#, main must be public.

So just think about it in real world scenarios.

E.g. you want to enter your room, but you should enter your home main door first (given room is inside the house...and no other means to enter the house)

Main door is the only publicly available access point.

回答9:

Because the main method should be accessed by JVM without any restrictions from any where. I think you can find more info here: Why main method is public