synchronized

[DKL01]

Hi,

I really appreciate if anybody can explain what's going on in getInstance method.

public class MLogin
{

private static MLogin instance = null;

public static MLogin getInstance (String address) throws exception
{
if (instance == null)
synchronized(Class("MLogin"))
{
if(instance == null)
instance = new MLogin();
instance.setValue(address);

}
}


public void setValue(address)
{
:
:
}

}

 

[sedj]

No sure where you got this code, but there are some fundamental errors in it :

- There is no class named "exception" - it is "Exception".
- The synchronized method is nonsense - it only makes sense to synch on an object such as "synchronized(this)" - but this would not be valid as the getInstance() method is static.
- There is no method called "Class" in the class.
- The setValue() method does not declare what kind of object "address" is.
- The getInstance() method does not return anything.

So to answer your question - the getInstance() method is not doing anything as it would not even compile ! If you fixed it, then it would return a static instance of the MLogin class - and all calls to it would return a reference to only one MLogin per JVM.

public class MLogin {

    private static MLogin instance = null;

    public static MLogin getInstance (String address) throws Exception {
		if(instance == null) {
			instance = new MLogin();
			instance.setValue(address);
		}

		return instance;

    }

    public void setValue(String address) {
        //:
        //:
    }

}

 

[DKL01]

While putting the code I made mistakes in syntax. Here is the code.

public class MLogin
{

private static MLogin instance = null;

public static MLogin getInstance (String address) throws Exception
{
if (instance == null)
synchronized(Class.forName("MLogin"))
{
if(instance == null)
instance = new MLogin();
instance.setValue(address);

}
}


public void setValue(String address)
{
//some code
}

}

 

[sedj]

Oh, well thanks for wasting my time anyway by posting incorrect code. The code you posted again will still not compile because it does not return anything.

As I said last time getInstance() is doing :

If you fixed it, then it would return a static instance of the MLogin class - and all calls to it would return a reference to only one MLogin per JVM.


Quite why you need to synchronize on the static initializer of MLogin I don't know - the JVM would handle any multi-threaded calls to this class method because it is static, and returns a static class (ie one per JVM).