深入学习java源码之System.console()与System.load()

深入学习java源码之System.console()与System.load()

System类包含几个有用的类字段和方法。 它不能被实例化。 
System类提供的System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; 一种加载文件和库的方法; 以及用于快速复制阵列的一部分的实用方法。 

方法

Modifier and Type	Method and Description
static void	arraycopy(Object src, int srcPos, Object dest, int destPos, int length) 将指定源数组中的数组从指定位置复制到目标数组的指定位置。
static String	clearProperty(String key) 删除指定键指定的系统属性。
static Console	console() 返回与当前Java虚拟机关联的唯一的Console对象（如果有）。
static long	currentTimeMillis() 返回当前时间（以毫秒为单位）。
static void	exit(int status) 终止当前运行的Java虚拟机。
static void	gc() 运行垃圾回收器。
static Map<String,String>	getenv() 返回当前系统环境的不可修改的字符串映射视图。
static String	getenv(String name) 获取指定环境变量的值。
static Properties	getProperties() 确定当前的系统属性。
static String	getProperty(String key) 获取指定键指示的系统属性。
static String	getProperty(String key, String def) 获取指定键指示的系统属性。
static SecurityManager	getSecurityManager() 获取系统安全界面。
static int	identityHashCode(Object x) 返回与默认方法hashCode（）返回的给定对象相同的哈希码，无论给定对象的类是否覆盖了hashCode（）。
static Channel	inheritedChannel() 返回从创建此Java虚拟机的实体继承的通道。
static String	lineSeparator() 返回与系统相关的行分隔符字符串。
static void	load(String filename) 加载由filename参数指定的本机库。
static void	loadLibrary(String libname) 加载 libname参数指定的本机库。
static String	mapLibraryName(String libname) 将库名称映射到表示本地库的平台特定字符串。
static long	nanoTime() 以纳秒为单位返回正在运行的Java虚拟机的高分辨率时间源的当前值。
static void	runFinalization() 运行任何对象等待定稿的最终化方法。
static void	runFinalizersOnExit(boolean value)已弃用 这种方法本质上是不安全的。 它可能导致在活动对象上调用finalizer，而其他线程同时操作这些对象，导致不稳定的行为或死锁。
static void	setErr(PrintStream err) 重新分配“标准”错误输出流。
static void	setIn(InputStream in) 重新分配“标准”输入流。
static void	setOut(PrintStream out) 重新分配“标准”输出流。
static void	setProperties(Properties props) 将系统属性设置为 Properties参数。
static String	setProperty(String key, String value) 设置由指定键指示的系统属性。
static void	setSecurityManager(SecurityManager s) 设置系统安全性。

java源码

package java.lang;

import java.io.*;
import java.lang.reflect.Executable;
import java.lang.annotation.Annotation;
import java.security.AccessControlContext;
import java.util.Properties;
import java.util.PropertyPermission;
import java.util.StringTokenizer;
import java.util.Map;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.security.AllPermission;
import java.nio.channels.Channel;
import java.nio.channels.spi.SelectorProvider;
import sun.nio.ch.Interruptible;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;
import sun.security.util.SecurityConstants;
import sun.reflect.annotation.AnnotationType;

public final class System {

    private static native void registerNatives();
    static {
        registerNatives();
    }

    private System() {
    }


    public final static InputStream in = null;

    public final static PrintStream out = null;

    public final static PrintStream err = null;

    private static volatile SecurityManager security = null;

    public static void setIn(InputStream in) {
        checkIO();
        setIn0(in);
    }

    public static void setOut(PrintStream out) {
        checkIO();
        setOut0(out);
    }

    public static void setErr(PrintStream err) {
        checkIO();
        setErr0(err);
    }

    private static volatile Console cons = null;

     public static Console console() {
         if (cons == null) {
             synchronized (System.class) {
                 cons = sun.misc.SharedSecrets.getJavaIOAccess().console();
             }
         }
         return cons;
     }

    public static Channel inheritedChannel() throws IOException {
        return SelectorProvider.provider().inheritedChannel();
    }

    private static void checkIO() {
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("setIO"));
        }
    }

    private static native void setIn0(InputStream in);
    private static native void setOut0(PrintStream out);
    private static native void setErr0(PrintStream err);

    public static
    void setSecurityManager(final SecurityManager s) {
        try {
            s.checkPackageAccess("java.lang");
        } catch (Exception e) {
            // no-op
        }
        setSecurityManager0(s);
    }

    private static synchronized
    void setSecurityManager0(final SecurityManager s) {
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            // ask the currently installed security manager if we
            // can replace it.
            sm.checkPermission(new RuntimePermission
                                     ("setSecurityManager"));
        }

        if ((s != null) && (s.getClass().getClassLoader() != null)) {
            // New security manager class is not on bootstrap classpath.
            // Cause policy to get initialized before we install the new
            // security manager, in order to prevent infinite loops when
            // trying to initialize the policy (which usually involves
            // accessing some security and/or system properties, which in turn
            // calls the installed security manager's checkPermission method
            // which will loop infinitely if there is a non-system class
            // (in this case: the new security manager class) on the stack).
            AccessController.doPrivileged(new PrivilegedAction<Object>() {
                public Object run() {
                    s.getClass().getProtectionDomain().implies
                        (SecurityConstants.ALL_PERMISSION);
                    return null;
                }
            });
        }

        security = s;
    }

    public static SecurityManager getSecurityManager() {
        return security;
    }

    public static native long currentTimeMillis();

    public static native long nanoTime();

    public static native void arraycopy(Object src,  int  srcPos,
                                        Object dest, int destPos,
                                        int length);

    public static native int identityHashCode(Object x);

    private static Properties props;
    private static native Properties initProperties(Properties props);

    public static Properties getProperties() {
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPropertiesAccess();
        }

        return props;
    }

    public static String lineSeparator() {
        return lineSeparator;
    }

    private static String lineSeparator;

    public static void setProperties(Properties props) {
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPropertiesAccess();
        }
        if (props == null) {
            props = new Properties();
            initProperties(props);
        }
        System.props = props;
    }

    public static String getProperty(String key) {
        checkKey(key);
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPropertyAccess(key);
        }

        return props.getProperty(key);
    }

    public static String getProperty(String key, String def) {
        checkKey(key);
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPropertyAccess(key);
        }

        return props.getProperty(key, def);
    }

    public static String setProperty(String key, String value) {
        checkKey(key);
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new PropertyPermission(key,
                SecurityConstants.PROPERTY_WRITE_ACTION));
        }

        return (String) props.setProperty(key, value);
    }

    public static String clearProperty(String key) {
        checkKey(key);
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new PropertyPermission(key, "write"));
        }

        return (String) props.remove(key);
    }

    private static void checkKey(String key) {
        if (key == null) {
            throw new NullPointerException("key can't be null");
        }
        if (key.equals("")) {
            throw new IllegalArgumentException("key can't be empty");
        }
    }

    public static String getenv(String name) {
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("getenv."+name));
        }

        return ProcessEnvironment.getenv(name);
    }

    public static java.util.Map<String,String> getenv() {
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("getenv.*"));
        }

        return ProcessEnvironment.getenv();
    }

    public static void exit(int status) {
        Runtime.getRuntime().exit(status);
    }

    public static void gc() {
        Runtime.getRuntime().gc();
    }

    public static void runFinalization() {
        Runtime.getRuntime().runFinalization();
    }

    @Deprecated
    public static void runFinalizersOnExit(boolean value) {
        Runtime.runFinalizersOnExit(value);
    }

    @CallerSensitive
    public static void load(String filename) {
        Runtime.getRuntime().load0(Reflection.getCallerClass(), filename);
    }

    @CallerSensitive
    public static void loadLibrary(String libname) {
        Runtime.getRuntime().loadLibrary0(Reflection.getCallerClass(), libname);
    }

    public static native String mapLibraryName(String libname);

    private static PrintStream newPrintStream(FileOutputStream fos, String enc) {
       if (enc != null) {
            try {
                return new PrintStream(new BufferedOutputStream(fos, 128), true, enc);
            } catch (UnsupportedEncodingException uee) {}
        }
        return new PrintStream(new BufferedOutputStream(fos, 128), true);
    }


    private static void initializeSystemClass() {

        // VM might invoke JNU_NewStringPlatform() to set those encoding
        // sensitive properties (user.home, user.name, boot.class.path, etc.)
        // during "props" initialization, in which it may need access, via
        // System.getProperty(), to the related system encoding property that
        // have been initialized (put into "props") at early stage of the
        // initialization. So make sure the "props" is available at the
        // very beginning of the initialization and all system properties to
        // be put into it directly.
        props = new Properties();
        initProperties(props);  // initialized by the VM

        // There are certain system configurations that may be controlled by
        // VM options such as the maximum amount of direct memory and
        // Integer cache size used to support the object identity semantics
        // of autoboxing.  Typically, the library will obtain these values
        // from the properties set by the VM.  If the properties are for
        // internal implementation use only, these properties should be
        // removed from the system properties.
        //
        // See java.lang.Integer.IntegerCache and the
        // sun.misc.VM.saveAndRemoveProperties method for example.
        //
        // Save a private copy of the system properties object that
        // can only be accessed by the internal implementation.  Remove
        // certain system properties that are not intended for public access.
        sun.misc.VM.saveAndRemoveProperties(props);


        lineSeparator = props.getProperty("line.separator");
        sun.misc.Version.init();

        FileInputStream fdIn = new FileInputStream(FileDescriptor.in);
        FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);
        FileOutputStream fdErr = new FileOutputStream(FileDescriptor.err);
        setIn0(new BufferedInputStream(fdIn));
        setOut0(newPrintStream(fdOut, props.getProperty("sun.stdout.encoding")));
        setErr0(newPrintStream(fdErr, props.getProperty("sun.stderr.encoding")));

        // Load the zip library now in order to keep java.util.zip.ZipFile
        // from trying to use itself to load this library later.
        loadLibrary("zip");

        // Setup Java signal handlers for HUP, TERM, and INT (where available).
        Terminator.setup();

        // Initialize any miscellenous operating system settings that need to be
        // set for the class libraries. Currently this is no-op everywhere except
        // for Windows where the process-wide error mode is set before the java.io
        // classes are used.
        sun.misc.VM.initializeOSEnvironment();

        // The main thread is not added to its thread group in the same
        // way as other threads; we must do it ourselves here.
        Thread current = Thread.currentThread();
        current.getThreadGroup().add(current);

        // register shared secrets
        setJavaLangAccess();

        // Subsystems that are invoked during initialization can invoke
        // sun.misc.VM.isBooted() in order to avoid doing things that should
        // wait until the application class loader has been set up.
        // IMPORTANT: Ensure that this remains the last initialization action!
        sun.misc.VM.booted();
    }

    private static void setJavaLangAccess() {
        // Allow privileged classes outside of java.lang
        sun.misc.SharedSecrets.setJavaLangAccess(new sun.misc.JavaLangAccess(){
            public sun.reflect.ConstantPool getConstantPool(Class<?> klass) {
                return klass.getConstantPool();
            }
            public boolean casAnnotationType(Class<?> klass, AnnotationType oldType, AnnotationType newType) {
                return klass.casAnnotationType(oldType, newType);
            }
            public AnnotationType getAnnotationType(Class<?> klass) {
                return klass.getAnnotationType();
            }
            public Map<Class<? extends Annotation>, Annotation> getDeclaredAnnotationMap(Class<?> klass) {
                return klass.getDeclaredAnnotationMap();
            }
            public byte[] getRawClassAnnotations(Class<?> klass) {
                return klass.getRawAnnotations();
            }
            public byte[] getRawClassTypeAnnotations(Class<?> klass) {
                return klass.getRawTypeAnnotations();
            }
            public byte[] getRawExecutableTypeAnnotations(Executable executable) {
                return Class.getExecutableTypeAnnotationBytes(executable);
            }
            public <E extends Enum<E>>
                    E[] getEnumConstantsShared(Class<E> klass) {
                return klass.getEnumConstantsShared();
            }
            public void blockedOn(Thread t, Interruptible b) {
                t.blockedOn(b);
            }
            public void registerShutdownHook(int slot, boolean registerShutdownInProgress, Runnable hook) {
                Shutdown.add(slot, registerShutdownInProgress, hook);
            }
            public int getStackTraceDepth(Throwable t) {
                return t.getStackTraceDepth();
            }
            public StackTraceElement getStackTraceElement(Throwable t, int i) {
                return t.getStackTraceElement(i);
            }
            public String newStringUnsafe(char[] chars) {
                return new String(chars, true);
            }
            public Thread newThreadWithAcc(Runnable target, AccessControlContext acc) {
                return new Thread(target, acc);
            }
            public void invokeFinalize(Object o) throws Throwable {
                o.finalize();
            }
        });
    }
}