QuickGuidetojava.lang.System
2. IO
System is a part of java.lang, and one of its main features is to give us access to the standard I/O streams.
Simply put, it exposes three fields, one for each stream:
- out
- err
- in
2.1. System.out
System.out points to the standard output stream, exposing it as a PrintStream, and we can use it to print text to the console:
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System.out.print(
"some inline message"
);
An advanced usage of System is to call System.setOut, which we can use to customize the location to which System.out will write:
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// Redirect to a text file
System.setOut(
new
PrintStream(
"filename.txt"
));
2.2. System.err
System.err is a lot like System.out. Both fields are instances of PrintStream, and both are for printing messages to the console.
But System.err represents standard error and we use that specifically to output error messages:
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System.err.print(
"some inline error message"
);
Consoles will often render the error stream differently than the output stream.
For more information, check the PrintStream documentation.
2.3. System.in
System.in points to the standard in, exposing it as an InputStream, and we can use it for reading input from the console.
And while a bit more involved, we can still manage:
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public
String readUsername(
int
length)
throws
IOException {
byte
[] name =
new
byte
[length];
System.in.read(name,
0
, length);
// by default, from the console
return
new
String(name);
}
By calling System.in.read, the application stops and waits for input from the standard in. Whatever the next length bytes are will be read from the stream and stored in the byte array.
Anything else typed by the user stays in the stream, waiting for another call to read.
Of course, operating at that low of a level can be challenging and error-prone, so we can clean it up a bit with BufferedReader:
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public
String readUsername()
throws
IOException {
BufferedReader reader =
new
BufferedReader(
new
InputStreamReader(System.in));
return
reader.readLine();
}
With the above arrangement, readLine will read from System.in until the user hits return, which is a bit closer to what we might expect.
Note that we purposely don"t close the stream in this case. Closing the standard in means that it cannot be read again for the lifecycle of the program!
And finally, an advanced usage of System.in is to call System.setIn to redirect it to a different InputStream.
3. Utility Methods
System provides us with numerous methods to help us with things like:
- Accessing the console
- Copying arrays
- Observing date and time
- Exiting the JRE
- Accessing runtime properties
- Accessing environment variables, and
- Administering garbage collection
3.1. Accessing the Console
Java 1.6 introduced another way of interacting with the console than simply using System.out and in directly.
We can access it by calling System.console:
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public
String readUsername() {
Console console = System.console();
return
console ==
null
?
null
:
console.readLine(
"%s"
,
"Enter your name: "
);
}
Note that depending upon the underlying operating system and how we launch Java to run the current program, console might return null, so always make sure to check before using.
Check out the Console documentation for more uses.
3.2. Copying Arrays
System.arraycopy is an old C-style way of copying one array into another.
Mostly, arraycopy is intended to copy one complete array into another array:
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int
[] a = {
34
,
22
,
44
,
2
,
55
,
3
};
int
[] b =
new
int
[a.length];
System.arraycopy(a,
0
, b,
0
, a.length);
assertArrayEquals(a, b);
However, we can specify the starting position for both arrays, as well as how many elements to copy.
For example, let"s say we want to copy 2 elements from a, starting at a[1] to b, starting at b[3]:
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System.arraycopy(a,
1
, b,
3
,
2
);
assertArrayEquals(
new
int
[] {
0
,
0
,
0
,
22
,
44
,
0
}, b);
Also, remember that arraycopy will throw:
- NullPointerException if either array is null
- IndexOutOfBoundsException if the copy references either array beyond its range
- ArrayStoreException if the copy results in a type mismatch
3.3. Observing Date and Time
There"re two methods related to time in System. One is currentTimeMillis and the other is nanoTime.
currentTimeMillis returns the number of milliseconds passed since the Unix Epoch, which is January 1, 1970 12:00 AM UTC:
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public
long
nowPlusOneHour() {
return
System.currentTimeMillis() +
3600
* 1000L;
}
public
String nowPrettyPrinted() {
return
new
Date(System.currentTimeMillis()).toString();
}
nanoTime returns the time relative to JVM startup. We can call it multiple times to mark the passage of time in the application:
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long
startTime = System.nanoTime();
// ...
long
endTime = System.nanoTime();
assertTrue(endTime - startTime <
10000
);
Note that since nanoTime is so fine-grained, it"s safer to do endTime – startTime < 10000 than endTime < startTime due to the possibility of numerical overflow.
3.4. Exiting the Program
If we want to programmatically exit the currently executed program, System.exit will do the trick.
To invoke exit, we need to specify an exit code, which will get sent to the console or shell that launched the program.
By convention in Unix, a status of 0 means a normal exit, while non-zero means some error occurred:
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if
(error) {
System.exit(
1
);
}
else
{
System.exit(
0
);
}
Note that for most programs nowadays, it"d be strange to need to call this. When called in a web server application, for example, it may take down the entire site!
3.5. Accessing Runtime Properties
System provides access to runtime properties with getProperty.
And we can manage them with setProperty and clearProperty:
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public
String getJavaVMVendor() {
System.getProperty(
"java.vm.vendor"
);
}
System.setProperty(
"abckey"
,
"abcvaluefoo"
);
assertEquals(
"abcvaluefoo"
, System.getProperty(
"abckey"
));
System.clearProperty(
"abckey"
);
assertNull(System.getProperty(
"abckey"
));
Properties specified via -D are accessible via getProperty.
We can also provide a default:
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System.clearProperty(
"dbHost"
);
String myKey = System.getProperty(
"dbHost"
,
"db.host.com"
);
assertEquals(
"db.host.com"
, myKey);
And System.getProperties provides a collection of all system properties:
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Properties properties = System.getProperties();
From which we can do any Properties operations:
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public
void
clearAllProperties() {
System.getProperties().clear();
}
3.6. Accessing Environment Variables
System also provides read-only access to environment variables with getenv.
If we want to access the PATH environment variable, for example, we can do:
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public
String getPath() {
return
System.getenv(
"PATH"
);
}
3.7. Administering Garbage Collection
Typically, garbage collection efforts are opaque to our programs. On occasion, though, we may want to make a direct suggestion to the JVM.
System.runFinalization is a method that allows us to suggest that the JVM run its finalize routine.
System.gc is a method that allows us to suggest that the JVM run its garbage collection routine.
Since contracts of these two methods don"t guarantee that finalization or garbage collection will run, their usefulness is narrow.
However, they could be exercised as an optimization, say invoking gc when a desktop app gets minimized:
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public
void
windowStateChanged(WindowEvent event) {
if
( event == WindowEvent.WINDOW_DEACTIVATED ) {
System.gc();
// if it ends up running, great!
}
}
For more on finalization, check out our finalize guide.
4. Conclusion
In this article, we got to see some of the fields and methods System provides. The complete list can be found in the official System documentation.
Also, check out all the examples in this article over on Github.
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