Java Java.lang.System用法及代码示例

System 类提供的设施包括标准输入、标准输出和错误输出流；访问外部定义的属性和环境变量；加载文件和库的方法；以及一种用于快速复制数组的一部分的实用方法。它扩展了对象类。

领域：

1. 公共静态最终InputStream：“standard” 输入流。该流已打开并准备好提供输入数据。通常，该流对应于键盘输入或主机环境或用户指定的另一个输入源。
2. 公共静态最终PrintStream输出：“standard” 输出流。该流已打开并准备好接受输出数据。通常，该流对应于显示输出或由主机环境或用户指定的另一个输出目的地。
3. 公共静态最终PrintStream错误：“standard” 错误输出流。该流已打开并准备好接受输出数据。
   通常，该流对应于显示输出或由主机环境或用户指定的另一个输出目的地。按照惯例，此输出流用于显示错误消息或其他应引起用户立即注意的信息，即使主要输出流(变量 out 的值)已重定向到文件或其他目标通常不会连续监测。

方法：

1. static void arraycopy(Object source, int sourceStart, Object Target, int targetStart, int size)：复制数组。要复制的数组在 source 中传递，并且在 sourceStart 中传递在 source 中开始复制的索引。将接收副本的数组在 target 中传递，并且在 targetStart 中传递副本将在目标内开始的索引。 Size 是复制的元素数量。

Syntax: public static void arraycopy(Object source, 
int sourceStart, Object Target, int targetStart, int size)
Returns: NA.
Exception: 
IndexOutOfBoundsException - if copying would cause access of data 
outside array bounds.
ArrayStoreException - if an element in the source array could not 
be stored into the target array because of a type mismatch.
NullPointerException - if either source or target is null.

// Java code illustrating arraycopy() method
import java.lang.*;
import java.util.Arrays;
class SystemDemo
{
    public static void main(String args[])
    {
        int[] a = {1, 2, 3, 4, 5};
        int[] b = {6, 7, 8, 9, 10};
         
        System.arraycopy(a, 0, b, 2, 2);
         
        // array b after arraycopy operation
        System.out.println(Arrays.toString(b));
         
    }
}

输出：

[6, 7, 1, 2, 10]

2. static StringclearProperty(String key)：删除指定键指示的系统属性。

Syntax: public static String clearProperty(String key)
Returns: the previous string value 
of the system property, or null if there was no property 
with that key.
Exception: 
SecurityException - if a security manager exists and its 
checkPropertyAccess method doesn't allow 
access to the specified system property.
NullPointerException - if key is null.
IllegalArgumentException - if key is empty.

3. static String getProperty(String key)：获取指定key指示的系统属性。

Syntax: public static String getProperty(String key)
Returns: the string value of the system 
property, or null if there is no property with that key.
Exception: 
SecurityException - if a security manager exists and its 
checkPropertyAccess method doesn't allow access to the 
specified system property.
NullPointerException - if key is null.
IllegalArgumentException - if key is empty.

4. static String getProperty(String key, String def)：获取指定key指示的系统属性。

Syntax: public static String getProperty(String key, String def)
Returns: the string value of the system property,
 or the default value if there is no property with that key.
Exception: 
SecurityException - if a security manager exists and its 
checkPropertyAccess method doesn't allow access to the 
specified system property.
NullPointerException - if key is null.
IllegalArgumentException - if key is empty.

5. static String setProperty(String key, String value)：设置指定键指示的系统属性。

Syntax: public static String setProperty(String key, String value)
Returns: the previous value of the system 
property, or null if it did not have one.
Exception: 
SecurityException - if a security manager exists and its checkPermission 
method doesn't allow setting of the specified property.
NullPointerException - if key or value is null.
IllegalArgumentException - if key is empty.

// Java code illustrating clearProperty(), getProperty()
// and setProperty() methods
import java.lang.*;
import static java.lang.System.clearProperty;
import static java.lang.System.setProperty;
import java.util.Arrays;
class SystemDemo
{
    public static void main(String args[])
    {
        // checking specific property
        System.out.println(System.getProperty("user.home"));
         
        // clearing this property
        clearProperty("user.home");
         
        System.out.println(System.getProperty("user.home"));
         
        // setting specific property
        setProperty("user.country", "US");
         
        // checking property
        System.out.println(System.getProperty("user.country"));
         
        // checking property other than system property
        // illustrating getProperty(String key, String def)
        System.out.println(System.getProperty("user.password",
              "none of your business"));
    }
}

输出：

/Users/abhishekverma
null
US
none of your business

6. static Console console()：返回与当前Java虚拟机关联的唯一Console对象(如果有)。

Syntax: public static Console console()
Returns: The system console, if any, otherwise null.
Exception: NA

// Java code illustrating console() method
import java.io.Console;
import java.lang.*;
import java.util.Currency;
import java.util.Locale;
class SystemDemo
{
    public static void main(String args[]) throws NullPointerException
    {
         
        Console c = System.console();
        if(c != null)
        {
           Currency currency = Currency.getInstance(Locale.ITALY);
           c.printf(currency.getSymbol());
            c.flush();
        }
        else
            System.out.println("No console attached");
    }
}

输出：

No console attached

7. static long currentTimeMillis()：返回当前时间(以毫秒为单位)。请注意，虽然返回值的时间单位是毫秒，但该值的粒度取决于底层操作系统，并且可能会更大。例如，许多操作系统以数十毫秒为单位测量时间。

Syntax: public static long currentTimeMillis()
Returns: the difference, measured in milliseconds,
 between the current time and midnight, January 1, 1970 UTC.
Exception: NA.

8. static long nanoTime()：返回正在运行的Java虚拟机的high-resolution时间源的当前值，以纳秒为单位。

Syntax: public static long nanoTime()
Returns: the current value of the running Java
 Virtual Machine's high-resolution time source, in nanoseconds
Exception: NA

// Java code illustrating currentTimeMillis() method
import java.lang.*;
class SystemDemo
{
    public static void main(String args[]) throws NullPointerException
    {
        System.out.println("difference between the "
                + "current time and midnight,"
                + " January 1, 1970 UTC is: " + 
                System.currentTimeMillis());
        System.out.println("current time in "
                + "nano sec: " + 
                System.nanoTime());
    }
}

输出：

difference between the current time 
and midnight, January 1, 1970 UTC is: 
1499520649545
current time in nano sec: 29976939759226

9.静态无效退出(int状态)：终止当前正在运行的 Java 虚拟机。该参数用作状态代码；按照惯例，非零状态代码表示异常终止。
该方法调用Runtime类中的exit方法。此方法永远不会正常返回。
调用 System.exit(n) 实际上等同于以下调用：
运行时.getRuntime().exit(n)

Syntax: public static void exit(int status)
Returns: NA
Exception: 
SecurityException - if a security manager exists and its 
checkExit method doesn't allow exit with the specified status.

// Java code illustrating exit() method 
import java.lang.*;
class SystemDemo
{
    public static void main(String args[]) throws NullPointerException
    {
        System.gc();
        System.out.println("Garbage collector executed ");
         
        System.out.println(System.getProperty("os.name"));
         
        System.exit(1);
         
        // this line will not execute as JVM terminated
        System.out.println("JVM terminated");
    }
}

输出：

Garbage collector executed 
Mac OS X

10. static void gc()：运行垃圾Collector。调用 gc 方法表明 Java 虚拟机将努力回收未使用的对象，以使它们当前占用的内存可供快速重用。当控制权从方法调用返回时，Java 虚拟机已尽最大努力从所有丢弃的对象中回收空间。

Syntax: public static void gc()
Returns: NA
Exception: NA

// Java code illustrating gc() method
import java.lang.*;
class SystemDemo
{
    public static void main(String args[]) 
    {
        Runtime gfg = Runtime.getRuntime();
        long memory1, memory2;
        Integer integer[] = new Integer[1000];
  
        // checking the total memory
        System.out.println("Total memory is: "
                           + gfg.totalMemory());
  
        // checking free memory
        memory1 = gfg.freeMemory();
        System.out.println("Initial free memory: "
                                      + memory1);
  
        // calling the garbage collector on demand
        System.gc();
  
        memory1 = gfg.freeMemory();
  
        System.out.println("Free memory after garbage "
                           + "collection: " + memory1);
  
        // allocating integers
        for (int i = 0; i < 1000; i++)
            integer[i] = new Integer(i);
  
        memory2 = gfg.freeMemory();
        System.out.println("Free memory after allocation: "
                           + memory2);
  
        System.out.println("Memory used by allocation: " +
                                    (memory1 - memory2));
  
        // discard integers
        for (int i = 0; i < 1000; i++)
            integer[i] = null;
  
        System.gc();
  
        memory2 = gfg.freeMemory();
        System.out.println("Free memory after  "
            + "collecting discarded Integers: " + memory2);
    }
}

输出：

Total memory is: 128974848
Initial free memory: 126929976
Free memory after garbage collection: 128632160
Free memory after allocation: 127950520
Memory used by allocation: 681640
Free memory after  collecting discarded Integers: 128643472

11.静态Mapgetenv()：返回当前系统环境的不可修改的字符串映射视图。环境是一个依赖于系统的从名称到值的映射，该映射从父进程传递到子进程。
如果系统不支持环境变量，则返回空映射。

Syntax: public static Map getenv()
Returns: the environment as a map of variable names to values.
Exception: 
SecurityException - if a security manager exists and its 
checkPermission method doesn't allow access to the process 
environment

12. static String getenv(字符串名称):获取指定环境变量的值。环境变量是依赖于系统的外部命名值。
系统属性和环境变量在概念上都是名称和值之间的映射。这两种机制都可用于将用户定义的信息传递给 Java 进程。环境变量具有更全局的影响，因为它们对定义它们的进程的所有后代都可见，而不仅仅是直接的 Java 子进程。它们在不同的操作系统上可能具有细微不同的语义，例如不区分大小写。由于这些原因，环境变量更有可能产生意想不到的副作用。最好尽可能使用系统属性。当需要全局效果或外部系统接口需要环境变量(例如 PATH)时，应使用环境变量。

Syntax: public static String getenv(String name)
Returns: the string value of the variable,
 or null if the variable is not defined in the system environment.
Exception: 
NullPointerException - if name is null
SecurityException - if a security manager exists and 
its checkPermission method doesn't allow access to the
 environment variable name.

// Java code illustrating getenv() method
import java.lang.*;
import java.util.Map;
import java.util.Set;
class SystemDemo
{
    public static void main(String args[]) 
    {
        Map<String, String> gfg = System.getenv();
        Set<String> keySet = gfg.keySet();
        for(String key : keySet)
        {
            System.out.println("key= " + key);
        }
         
        // checking specific environment variable
        System.out.println(System.getenv("PATH"));
    }
}

输出：

key= JAVA_MAIN_CLASS_5396
key= PATH
key= J2D_PIXMAPS
key= SHELL
key= USER
key= TMPDIR
key= SSH_AUTH_SOCK
key= XPC_FLAGS
key= LD_LIBRARY_PATH
key= __CF_USER_TEXT_ENCODING
key= Apple_PubSub_Socket_Render
key= LOGNAME
key= LC_CTYPE
key= XPC_SERVICE_NAME
key= PWD
key= JAVA_MAIN_CLASS_2336
key= SHLVL
key= HOME
key= _
/usr/bin:/bin:/usr/sbin:/sbin

13. static Properties getProperties()：确定当前系统属性。

Syntax: public static Properties getProperties()
Returns: the system properties.
Exception: 
SecurityException - if a security manager exists and 
its checkPropertiesAccess method doesn't allow access 
to the system properties.

// Java code illustrating getProperties() method
import java.lang.*;
import java.util.Properties;
import java.util.Set;
class SystemDemo
{
    public static void main(String args[]) 
    {
        Properties gfg = System.getProperties();
        Set<Object> keySet = gfg.keySet();
        for(Object key : keySet)
        {
            System.out.println("key= " + key);
        }
    }
}

输出：

key= java.runtime.name
key= sun.boot.library.path
key= java.vm.version
key= user.country.format
key= gopherProxySet
key= java.vm.vendor
key= java.vendor.url
key= path.separator
key= java.vm.name
key= file.encoding.pkg
key= user.country
key= sun.java.launcher
key= sun.os.patch.level
key= java.vm.specification.name
key= user.dir
key= java.runtime.version
key= java.awt.graphicsenv
key= java.endorsed.dirs
key= os.arch
key= java.io.tmpdir
key= line.separator
key= java.vm.specification.vendor
key= os.name
key= sun.jnu.encoding
key= java.library.path
key= java.specification.name
key= java.class.version
key= sun.management.compiler
key= os.version
key= http.nonProxyHosts
key= user.home
key= user.timezone
key= java.awt.printerjob
key= file.encoding
key= java.specification.version
key= java.class.path
key= user.name
key= java.vm.specification.version
key= sun.java.command
key= java.home
key= sun.arch.data.model
key= user.language
key= java.specification.vendor
key= awt.toolkit
key= java.vm.info
key= java.version
key= java.ext.dirs
key= sun.boot.class.path
key= java.vendor
key= file.separator
key= java.vendor.url.bug
key= sun.io.unicode.encoding
key= sun.cpu.endian
key= socksNonProxyHosts
key= ftp.nonProxyHosts
key= sun.cpu.isalist

14.静态SecurityManager getSecurityManager()：获取系统安全接口。

Syntax: static SecurityManager getSecurityManager()
Returns: if a security manager has 
already been established for the current application,
 then that security manager is returned; otherwise, 
null is returned.
Exception: NA

15.静态无效setSecurityManager(SecurityManager s)：设置系统安全性。

Syntax: public static void setSecurityManager(SecurityManager s)
Returns: NA.
Exception: 
SecurityException - if the security manager has 
already been set and its checkPermission method 
doesn't allow it to be replaced.

// Java code illustrating setSecurityManager() 
// and getSecurityManager() method
import java.lang.*;
class SystemDemo
{
    public static void main(String args[]) 
    {
        SecurityManager gfg = new SecurityManager();
         
        // setting the security manager
        System.setSecurityManager(gfg);
         
        gfg = System.getSecurityManager();
        if(gfg != null)
            System.out.println("Security manager is configured");
    }
}

输出：

Security manager is configured

16. static void setErr(PrintStream err)：重新分配“standard”错误输出流。

Syntax: public static void setErr(PrintStream err)
Returns: NA
Exception: 
SecurityException - if a security manager exists and its
 checkPermission method doesn't allow reassigning of the
 standard error output stream.

17. static void setIn(InputStream in)：重新分配“standard”输入流。

Syntax: public static void setIn(InputStream in)
Returns: NA.
Exception: 
SecurityException - if a security manager exists and its
 checkPermission method doesn't allow reassigning of the
 standard input stream.

18. static void setOut(PrintStream out)：重新分配“standard”输出流。

Syntax: public void setOut(PrintStream out)
Returns: NA
Exception: 
SecurityException - if a security manager exists and its
 checkPermission method doesn't allow reassigning of the
 standard output stream.

// Java code illustrating setOut(), setIn() and setErr() method
import java.lang.*;
import java.util.Properties;
import java.io.*;
class SystemDemo
{
    public static void main(String args[])  throws IOException  
    {
        FileInputStream IN = new FileInputStream("input.txt");
        FileOutputStream OUT = new FileOutputStream("system.txt");
         
        // set input stream
        System.setIn(IN);
        char c = (char) System.in.read();
        System.out.print(c);
         
        // set output stream
        System.setOut(new PrintStream(OUT));
        System.out.write("Hi Abhishek\n".getBytes());
         
        // set error stream
        System.setErr(new PrintStream(OUT));
        System.err.write("Exception message\n".getBytes());
    }
}

输出：上述java代码的输出取决于“input.txt”文件中的内容。
创建您自己的“input.txt”，然后运行代码并检查输出。

19. static void load(String filename)：从本地文件系统加载指定文件名的代码文件作为动态库。文件名参数必须是完整的路径名。

Syntax: public static void load(String filename)
Returns: NA
Exception: 
SecurityException - if a security manager exists and
 its checkLink method doesn't allow loading of the specified
 dynamic library
UnsatisfiedLinkError - if the file does not exist.
NullPointerException - if filename is null

20. static void loadLibrary(String libname)：加载libname参数指定的系统库。库名称映射到实际系统库的方式取决于系统。

Syntax: public static void loadLibrary(String libname)
Returns: NA
Exception: 
SecurityException - if a security manager exists and its 
checkLink method doesn't allow loading of the specified dynamic
 library
UnsatisfiedLinkError - if the library does not exist.
NullPointerException - if libname is null

21. static String mapLibraryName(String libname)：将库名称映射到表示本机库的特定于平台的字符串。

Syntax: public static String mapLibraryName(String libname)
Returns: a platform-dependent native library name.
Exception: NullPointerException - if libname is null

22. static void runFinalization()：运行任何待终结的对象的终结方法。调用此方法表明 Java 虚拟机将努力运行那些已发现已被丢弃但尚未运行 Finalize 方法的对象的 Finalize 方法。当控制权从方法调用返回时，Java 虚拟机已尽最大努力完成所有未完成的终结。

Syntax: public static void runFinalization()
Returns: NA
Exception: NA.

// Java code illustrating runFinalization(), load()
// loadLibrary() and mapLibraryName() method
import java.lang.*;
class SystemDemo
{
    public static void main(String args[]) throws NullPointerException
    {
  
        // map library name
        String libName = System.mapLibraryName("os.name");
        System.out.println("os.name library= " + libName);
  
        //load external libraries
        System.load("lixXYZ.so");
        System.loadLibrary("libos.name.dylib");
  
        //run finalization
        System.runFinalization();
  
    }
}

输出：

os.name library= libos.name.dylib

23. static int IdentityHashCode(Object x)：为给定对象返回与默认方法hashCode()返回的哈希码相同的哈希码，无论给定对象的类是否覆盖hashCode()。空引用的哈希码为零。

Syntax: public static int identityHashCode(Object x)
Returns: the hashCode.
Exception: NA.

24. static Channel inheritedChannel()：返回从创建此Java虚拟机的实体继承的通道。

Syntax: public static Channel inheritedChannel().
Returns:  inherited channel, if any, otherwise null.
Exception: 
IOException - If an I/O error occurs
SecurityException - If a security manager is present and
 it does not permit access to the channel.

25. static String lineSeparator()：返回系统相关的行分隔符字符串。它始终返回相同的值 - 系统属性行分隔符的初始值。

Syntax: public static String lineSeparator()
Returns: On UNIX systems, it returns "\n";
 on Microsoft Windows systems it returns "\r\n".
Exception: NA

// Java code illustrating lineSeparator(), inherentChannel()
// and identityHashCode() method
import java.io.IOException;
import java.lang.*;
import java.nio.channels.Channel;
class SystemDemo
{
    public static void main(String args[]) 
            throws NullPointerException,
            IOException
    {
  
        Integer x = 400;
        System.out.println(System.identityHashCode(x));
         
        Channel ch = System.inheritedChannel();
        System.out.println(ch);
         
        System.out.println(System.lineSeparator());
         
    }
}

输出：

1735600054
null
"\r\n"