优先级
线程的优先级（Priority）告诉调试程序该线程的重要程度有多大。假如有大量线程都被堵塞，都在等候运行，调试程序会首先运行具有最高优先级的那个线程。然而，这并不表示优先级较低的线程不会运行（换言之，不会因为存在优先级而导致死锁）。若线程的优先级较低，只不过表示它被准许运行的机会小一些而已。
可用getPriority()方法读取一个线程的优先级，并用setPriority()改变它。在下面这个程序片中，大家会发现计数器的计数速度慢了下来，因为它们关联的线程分配了较低的优先级：
//: Counter5.Java
// Adjusting the priorities of threads
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
class Ticker2 extends Thread {
private Button
b = new Button("Toggle"),
incPriority = new Button("up"),
decPriority = new Button("down");
private TextField
t = new TextField(10),
pr = new TextField(3); // Display priority
private int count = 0;
private boolean runFlag = true;
public Ticker2(Container c) {
b.addActionListener(new ToggleL());
incPriority.addActionListener(new UpL()); decPriority.addActionListener(new DownL());
Panel p = new Panel();
p.add(t);
p.add(pr);
p.add(b);
p.add(incPriority);
p.add(decPriority);
c.add(p);
}
class ToggleL implements ActionListener {
public void actionPerformed(ActionEvent e) {
runFlag = !runFlag;
}
}
class UpL implements ActionListener {
public void actionPerformed(ActionEvent e) {
int newPriority = getPriority() + 1;
if(newPriority > Thread.MAX_PRIORITY)
newPriority = Thread.MAX_PRIORITY;
setPriority(newPriority);
}
}
class DownL implements ActionListener {
public void actionPerformed(ActionEvent e) {
int newPriority = getPriority() - 1;
if(newPriority < Thread.MIN_PRIORITY)
newPriority = Thread.MIN_PRIORITY;
setPriority(newPriority);
}
}
public void run() {
while (true) {
if(runFlag) {
t.setText(Integer.toString(count++));
pr.setText(
Integer.toString(getPriority()));
}
yield();
}
}
}
public class Counter5 extends Applet {
private Button
start = new Button("Start"),
upMax = new Button("Inc Max Priority"),
downMax = new Button("Dec Max Priority");
private boolean started = false;
private static final int SIZE = 10;
private Ticker2[] s = new Ticker2[SIZE];
private TextField mp = new TextField(3);
public void init() {
for(int i = 0; i < s.length; i++)
s[i] = new Ticker2(this);
add(new Label("MAX_PRIORITY = "
+ Thread.MAX_PRIORITY));
add(new Label("MIN_PRIORITY = "
+ Thread.MIN_PRIORITY));
add(new Label("Group Max Priority = "));
add(mp);
add(start);
add(upMax); add(downMax);
start.addActionListener(new StartL());
upMax.addActionListener(new UpMaxL());
downMax.addActionListener(new DownMaxL());
showMaXPriority();
// Recursively display parent thread groups:
ThreadGroup parent =
s[0].getThreadGroup().getParent();
while(parent != null) {
add(new Label(
"Parent threadgroup max priority = "
+ parent.getMaxPriority()));
parent = parent.getParent();
}
}
public void showMaxPriority() {
mp.setText(Integer.toString(
s[0].getThreadGroup().getMaxPriority()));
}
class StartL implements ActionListener {
public void actionPerformed(ActionEvent e) {
if(!started) {
started = true;
for(int i = 0; i < s.length; i++)
s[i].start();
}
}
}
class UpMaxL implements ActionListener {
public void actionPerformed(ActionEvent e) {
int maxp =
s[0].getThreadGroup().getMaxPriority();
if(++maxp > Thread.MAX_PRIORITY)
maxp = Thread.MAX_PRIORITY;
s[0].getThreadGroup().setMaxPriority(maxp);
showMaxPriority();
}
}
class DownMaxL implements ActionListener {
public void actionPerformed(ActionEvent e) {
int maxp =
s[0].getThreadGroup().getMaxPriority();
if(--maxp < Thread.MIN_PRIORITY)
maxp = Thread.MIN_PRIORITY;
s[0].getThreadGroup().setMaxPriority(maxp);
showMaxPriority();
}
}
public static void main(String[] args) {
Counter5 applet = new Counter5();
Frame aFrame = new Frame("Counter5");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(300, 600);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
Ticker采用本章前面构造好的形式，但有一个额外的TextField（文本字段），用于显示线程的优先级；以及两个额外的按钮，用于人为提高及降低优先级。
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