考虑在方法中声明的对象:
public void foo() {
final Object obj = new Object();
// A long run job that consumes tons of memory and
// triggers garbage collection
}
obj 会被垃圾回收吗beforefoo() 返回?
UPDATE:
之前我以为obj不会被垃圾回收untilfoo() 返回。
然而,今天我发现自己错了。
我花了几个小时修复一个错误,最后发现问题是由 obj 垃圾收集引起的!
谁能解释为什么会发生这种情况?如果我想要固定 obj 如何实现?
这是有问题的代码。
public class Program
{
public static void main(String[] args) throws Exception {
String connectionString = "jdbc:mysql://<whatever>";
// I find wrap is gc-ed somewhere
SqlConnection wrap = new SqlConnection(connectionString);
Connection con = wrap.currentConnection();
Statement stmt = con.createStatement(ResultSet.TYPE_FORWARD_ONLY,
ResultSet.CONCUR_READ_ONLY);
stmt.setFetchSize(Integer.MIN_VALUE);
ResultSet rs = stmt.executeQuery("select instance_id, doc_id from
crawler_archive.documents");
while (rs.next()) {
int instanceID = rs.getInt(1);
int docID = rs.getInt(2);
if (docID % 1000 == 0) {
System.out.println(docID);
}
}
rs.close();
//wrap.close();
}
}
Java程序运行后,在崩溃之前会打印以下消息:
161000
161000
********************************
Finalizer CALLED!!
********************************
********************************
Close CALLED!!
********************************
162000
Exception in thread "main" com.mysql.jdbc.exceptions.jdbc4.CommunicationsException:
这是 SqlConnection 类的代码:
class SqlConnection
{
private final String connectionString;
private Connection connection;
public SqlConnection(String connectionString) {
this.connectionString = connectionString;
}
public synchronized Connection currentConnection() throws SQLException {
if (this.connection == null || this.connection.isClosed()) {
this.closeConnection();
this.connection = DriverManager.getConnection(connectionString);
}
return this.connection;
}
protected void finalize() throws Throwable {
try {
System.out.println("********************************");
System.out.println("Finalizer CALLED!!");
System.out.println("********************************");
this.close();
} finally {
super.finalize();
}
}
public void close() {
System.out.println("********************************");
System.out.println("Close CALLED!!");
System.out.println("********************************");
this.closeConnection();
}
protected void closeConnection() {
if (this.connection != null) {
try {
connection.close();
} catch (Throwable e) {
} finally {
this.connection = null;
}
}
}
}