Runtime

Runtime

The Runtime class encapsulates the run-time environment. You cannot instantiate a Runtime object. However, you can get a reference to the current Runtime object by calling the static method Runtime.getRuntime( ). Once you obtain a reference to the current Runtime object, you can call several methods that control the state and behavior of the Java Virtual Machine. Applets and other untrusted code typically cannot call any of the Runtime methods without raising a SecurityException. Several commonly used methods defined by Runtime are shown in Table 17-11.

Let’s look at two of the most common uses of the Runtime class: memory management and executing additional processes.

Memory Management

Although Java provides automatic garbage collection, sometimes you will want to know how large the object heap is and how much of it is left. You can use this information, for example, to check your code for efficiency or to approximate how many more objects of a certain type can be instantiated. To obtain these values, use the totalMemory( ) and freeMemory( ) methods.

As mentioned in Part I, Java’s garbage collector runs periodically to recycle unused objects. However, sometimes you will want to collect discarded objects prior to the collector’s next appointed rounds. You can run the garbage collector on demand by calling the gc( ) method. A good thing to try is to call gc( ) and then call freeMemory( ) to get a baseline memory usage. Next, execute your code and call freeMemory( ) again to see how much memory it is allocating. The following program illustrates this idea:

// Demonstrate totalMemory(), freeMemory() and gc().

class MemoryDemo {

public static void main(String args[]) { Runtime r = Runtime.getRuntime(); long mem1, mem2;

Integer someints[] = new Integer[1000];

System.out.println("Total memory is: " + r.totalMemory());

mem1 = r.freeMemory();

System.out.println("Initial free memory: " + mem1); r.gc();

mem1 = r.freeMemory();

System.out.println("Free memory after garbage collection: " + mem1);

for(int i=0; i<1000; i++)

someints[i] = new Integer(i); // allocate integers

mem2 = r.freeMemory();

System.out.println("Free memory after allocation: " + mem2);

System.out.println("Memory used by allocation: " + (mem1-mem2));

// discard Integers

for(int i=0; i<1000; i++) someints[i] = null; r.gc(); // request garbage collection

mem2 = r.freeMemory();

System.out.println("Free memory after collecting" + " discarded Integers: " + mem2);

}

}

Sample output from this program is shown here (of course, your actual results may vary):

Total memory is: 1048568

Initial free memory: 751392

Free memory after garbage collection: 841424

Free memory after allocation: 824000

Memory used by allocation: 17424

Free memory after collecting discarded Integers: 842640

Executing Other Programs

In safe environments, you can use Java to execute other heavyweight processes (that is, programs) on your multitasking operating system. Several forms of the exec( ) method allow you to name the program you want to run as well as its input parameters. The exec( ) method returns a Process object, which can then be used to control how your Java program interacts with this new running process. Because Java can run on a variety of platforms and under a variety of operating systems, exec( ) is inherently environment-dependent.

The following example uses exec( ) to launch notepad, Windows’ simple text editor. Obviously, this example must be run under the Windows operating system.

// Demonstrate exec().

class ExecDemo {

public static void main(String args[]) { Runtime r = Runtime.getRuntime(); Process p = null;

try {

p = r.exec("notepad");

} catch (Exception e) { System.out.println("Error executing notepad.");

}

}

}

There are several alternative forms of exec( ), but the one shown in the example is the most common. The Process object returned by exec( ) can be manipulated by Process’ methods after the new program starts running. You can kill the subprocess with the destroy( ) method. The waitFor( ) method causes your program to wait until the subprocess finishes. The exitValue( ) method returns the value returned by the subprocess when it is finished. This is typically 0 if no problems occur. Here is the preceding exec( ) example modified to wait for the running process to exit:

// Wait until notepad is terminated.

class ExecDemoFini {

public static void main(String args[]) { Runtime r = Runtime.getRuntime(); Process p = null;

try {

p = r.exec("notepad"); p.waitFor();

} catch (Exception e) { System.out.println("Error executing notepad.");

}

System.out.println("Notepad returned " + p.exitValue());

}

}