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It

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is

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critical

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to

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ensure

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that

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threads

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are

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started correctly.

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Thread

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startup can

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be

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misleading

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because

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sometimes

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the

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code

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appears

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to

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be

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performing

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the

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function

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correctly, when in fact it may be executing in the wrong thread.

The Thread.start() method starts executing a thread's run() method in that thread. It is a mistake to directly invoke the run() method on a Thread object. When invoked directly, the statements in the run() method execute in the current thread instead of the newly created thread. Furthermore, if the Thread object is not constructed from a Runnable object but rather by instantiating a subclass of Thread that does not override the run() method, a call to the subclass's run() method invokes Thread.run() which performs a no-op.

Noncompliant Code Example

This noncompliant code example explicitly invokes run() in the context of the current thread.

Code Block

bgColor	#FFCCCC

 whereas it may be operating in the presence of subtle concurrency issues.

It is usually a mistake to invoke the {{run()}} method on a {{Thread}} object. When invoked directly, the statements in the {{run()}} method execute, however, in the current thread instead of the newly created thread. Furthermore, if the {{Thread}} object is not constructed from a {{Runnable}} object but by instantiating a subclass of {{Thread}} that does not override the {{run()}} method, a call to the subclass's {{run()}} method invokes {{Thread.run()}} which does not perform any useful operations.

{mc}
// Hidden text: What does this line mean? 
It is recommended that if you have a {{Thread}} object that extends {{Runnable}}, and you wish to execute the object's {{run()}} method in the current thread, first cast the object to a {{Runnable}} and then invoke {{run()}}.
{mc}

h2. Noncompliant Code Example

The {{run()}} method of interface {{Runnable}} must be invoked in its own thread, however, this noncompliant code example explicitly invokes it in the context of the current thread. 

{code:bgColor=#FFCCCC}
class Foo implements Runnable {
  public void run() {
    // ...
  }
  
  public static void main(String[] args) {
    Foo f = new foo();
    new Thread(f).run();
  }
}
{code}

The {{

The start()

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method

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is

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not

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invoked

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on

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the

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new

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thread

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because

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of

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the

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incorrect

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assumption

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that

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run()

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starts the new thread.

...

Consequently,

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the

...

statements

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in

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the

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run()

...

method

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execute

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in

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the

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same

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thread

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instead

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of

...

the

...

new

...

one.

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Compliant

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Solution

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This

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compliant

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solution

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correctly

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uses

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the

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start()

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method

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to

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start

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a

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new

...

thread

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which

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then

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executes

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the

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run()

...

method.

{:=

Code Block

bgColor

	#ccccff

}
class Foo implements Runnable {
  public void run() {
    // ...
  }
  
  public static void main(String[] args) {
    Foo f = new foo();
    new Thread(f).start();
  }
}
{code}

h2. Exceptions

*EX1*: The {{Thread.

Exceptions

EX1: The run()

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method

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may

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be

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invoked when

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unit

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testing

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its functionality. Note that a class cannot be tested for multithreaded use by invoking run().

EX2: When using a Thread object that implements Runnable, and you wish to execute the object's run() method in the current thread, first cast the object to a Runnable and then invoke run().

Code Block

bgColor	#ccccff


Thread t = new Thread(new Runnable() {
    public void run() {
      // ...
    }
  });
t.run();               // bad, user prob meant t.start()
((Runnable) t).run();  // OK

Casting a thread to a Runnable before calling run() serves to document the intention of explicitly calling Thread.run().