學(xué)習(xí)筆記，僅供參考，有錯(cuò)必糾

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文章目錄

• • 多線程
  • • 設(shè)置線程的優(yōu)先級(jí)
    • 同步代碼塊
    • • 什么情況下需要同步
      • 同步代碼塊
    • 死鎖

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多線程

設(shè)置線程的優(yōu)先級(jí)

設(shè)置線程優(yōu)先級(jí)：

package com.guiyang.restudy3;public class D5GetThreadObject {public static void main(String[] args) {Thread t1 = new Thread(){public void run() {for(int i = 0; i < 10; i++) {System.out.println(getName() + "...xxxxxxxxxx" );}}};Thread t2 = new Thread(){public void run() {for(int i = 0; i < 10; i++) {System.out.println(getName() + "...yyyyy" );}}};Thread t3 = new Thread(){public void run() {for(int i = 0; i < 10; i++) {System.out.println(getName() + "...zz" );}}};t1.setPriority(Thread.MIN_PRIORITY); //設(shè)置最小的線程優(yōu)先級(jí)t2.setPriority(5);t3.setPriority(Thread.MAX_PRIORITY); //設(shè)置最大的線程優(yōu)先級(jí)t1.start();t2.start();t3.start();}}

控制臺(tái)輸出：

Thread-2...zz Thread-2...zz Thread-2...zz Thread-2...zz Thread-2...zz Thread-2...zz Thread-0...xxxxxxxxxx Thread-1...yyyyy Thread-1...yyyyy Thread-1...yyyyy Thread-1...yyyyy Thread-1...yyyyy Thread-1...yyyyy Thread-1...yyyyy Thread-0...xxxxxxxxxx Thread-2...zz Thread-2...zz Thread-2...zz Thread-2...zz Thread-0...xxxxxxxxxx Thread-1...yyyyy Thread-0...xxxxxxxxxx Thread-0...xxxxxxxxxx Thread-1...yyyyy Thread-1...yyyyy Thread-0...xxxxxxxxxx Thread-0...xxxxxxxxxx Thread-0...xxxxxxxxxx Thread-0...xxxxxxxxxx Thread-0...xxxxxxxxxx

可以看到優(yōu)先級(jí)高的線程2優(yōu)先輸出，緊接著是線程1和線程0.

同步代碼塊

什么情況下需要同步

當(dāng)多線程并發(fā)，有多段代碼同時(shí)執(zhí)行時(shí)，我們希望某一段代碼執(zhí)行的過程中CPU不要切換到其他線程，這時(shí)就需要同步；

如果兩段代碼是同步的, 那么同一時(shí)間只能執(zhí)行一段, 在一段代碼沒執(zhí)行結(jié)束之前, 不會(huì)執(zhí)行另外一段代碼.

同步代碼塊

使用synchronized關(guān)鍵字加上一個(gè)鎖對(duì)象來定義一段代碼，這就叫同步代碼塊；多個(gè)同步代碼塊如果使用相同的鎖對(duì)象, 那么他們就是同步的。

• 舉個(gè)例子

設(shè)置同步代碼塊：

package com.guiyang.restudy3;public class D5GetThreadObject {public static void main(String[] args) {final Printer p = new Printer();new Thread() {public void run() {while(true) {p.print1();}}}.start();new Thread() {public void run() {while(true) {p.print2();}}}.start();} }class Printer {Demo2 d = new Demo2();public void print1() {synchronized(d) { //同步代碼塊,鎖機(jī)制,鎖對(duì)象可以是任意的System.out.print("安徽");System.out.print("財(cái)經(jīng)");System.out.print("大學(xué)");System.out.print("\r\n");}}public void print2() {synchronized(d) { System.out.print("統(tǒng)計(jì)");System.out.print("與");System.out.print("應(yīng)用數(shù)學(xué)");System.out.print("學(xué)院");System.out.print("\r\n");}} }class Demo2 {}

在靜態(tài)方法上設(shè)置synchronized關(guān)鍵字，并且將字節(jié)碼對(duì)象作為鎖：

package com.guiyang.restudy3;public class D5GetThreadObject {public static void main(String[] args) {new Thread() {public void run() {while(true) {Printer.print1();}}}.start();new Thread() {public void run() {while(true) {Printer.print2();}}}.start();} }class Printer {public static synchronized void print1() {System.out.print("安徽");System.out.print("財(cái)經(jīng)");System.out.print("大學(xué)");System.out.print("\r\n");}public static void print2() {synchronized(Printer.class) { System.out.print("統(tǒng)計(jì)");System.out.print("與");System.out.print("應(yīng)用數(shù)學(xué)");System.out.print("學(xué)院");System.out.print("\r\n");}} }

死鎖

多線程同步的時(shí)候，如果出現(xiàn)同步代碼嵌套，使用相同鎖，則可能出現(xiàn)死鎖，比如：

package com.guiyang.restudy3;public class D5GetThreadObject {private static String s1 = "安財(cái)";private static String s2 = "數(shù)理統(tǒng)計(jì)";public static void main(String[] args) {new Thread() {public void run() {while(true) {synchronized(s1) {System.out.println(getName() + "...獲取" + s1 + "等待" + s2);synchronized(s2) {System.out.println(getName() + "...拿到" + s2);}}}}}.start();new Thread() {public void run() {while(true) {synchronized(s2) {System.out.println(getName() + "...獲取" + s2 + "等待" + s1);synchronized(s1) {System.out.println(getName() + "...拿到" + s1);}}}}}.start();} }

控制臺(tái)輸出：

可以看到，程序被鎖死，控制臺(tái)輸出在Thread-1...獲取數(shù)理統(tǒng)計(jì)等待安財(cái)就停止了。

總結(jié)

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